I agree with the OP, and I have been using this kind of setup for my X79 rig for over a year.
First, I am a huge believer in making one's storage subsystem as logically parallel as possible so as to be able to leverage the most possible performance under the vast majority of use scenarios. By logically, I mean that it should be built around your usage patterns, tailored to your needs.
I have been constantly updating, upgrading, and changing the"second ary" components in the rig for the same period of time... Additional SSD ('s), additional HDD's, upgrading (switching) the slowest or smallest drive at the point in time to a new faster and/or larger one, experiments regarding all kinds of arrays of many different disk capacity (R0/R1 2-drive up to 13-drive R60E), experimenting with short-stroking to various capacity a nd leaving the r est of the drive as an active partition for rarely accessed data (installers, media that I have not usedin 6mo or mmore but which hasn't met the 10-12mo point at which it is kept on my backups server), a variety of RAID solutions from onboard to discrete Hardware RoC Controller cards (from simple and inexpensive to the new PCIe3.0 SAS12Gbs models, including the Areca 1882xi-16/-24 w it's extremely fast dual core RAID chip, upgraded from 1 to 4GB DDR3-1600 low-latency cache, the number of drives supported directly ie w/o expanders is impressive, full support for SSD's in multiple ways (SSD RAID to caching platter arrays), immensely customizable features, and quite a bit more; the LSI 9360/9370/9380 series is also vvery impressive and my 9371-8i PCIe3.0 SAS12Gbs with CacheCade/CacheVault and battery backup+NAND Flash backup is also an amazing piece of hardware; three friends and I all used our total of 11 Samsung 840Pro 256GB drives and the Areca hit the max possible bandwidth and showed incredible scaling as well as perhaps the most impressive "non-interference" I have seen meaning that the access times and such were within margin of error of a single direct to Intel chipset single drive; the cards both have a lot of cache, but while Areca has 4GB of slower mmemory the LSI has 1GB of significantly faster (bandwidth and latency) DDR3-1866 8-9-9, and both leverage the cache so well that any file/data equal to or smaller than the cache sees a tremendous speed increase, to the point of fully saturating the bandwidth).
There are other things that I have tried it continue to use, so that's far from a complete list. RAMDisks, for example, using 6 of 16GB of my Ripjaws Z 2133c9 @ 2540 9-12-10, or the same amount/ratio with Trident X 2400C9 @ 2666 10-11-11, or 12 of 32GB Ripjaws Z 2400C9... When the RAMDisk is set as persistent (saved to drive on shutdown and loaded at startup), I eliminated the increase in boot times by simply having the image written to and read from a pair of X25 SLC 80GB I got free, in RAID0, and the boot up time was the same when in RAID1 but the shutdown time took a hit, which was negated (mostly) by switching to pair of 840Pro 128GB in R1 instead, with the incredible speeds of the Samsungs trumping the sheer indestructible-ness of the SLC NAND in this case, and while I leave my computer runningmost all of the time, and thus even the extra 28pct OP on the 840P's is more for consistent performance rather than endurance, I certainly see the benefits of SLC if you are constantly fully rewriting the RAMDisk.
The point, I love messing with my storage setup, tweaking and improving bit by bit, because likemmost modern computers that don't live in a giant room of a Datacenter, the storage subsystem is the most significant bottleneck in the machine.
SSD's are wonderful, brilliant tech that are unquestionably the best (only) way to go for the OS, and if you frequently (multiple times weekly) use a program that is random IO heavy, it will benefit from the ultra low latency and orders of magnitude greater 4K speeds compared to even the fastest hard drive. I have a Plextor M5P-Xtreme 256GB that holds all of my audio production programs and other similar programs, and instead of taking a minute or longer, I have never waited more than maybe 3-5sec for the most complex of the programs to fully load.
THE PREMISE AND WHY YOU WANT MORE DRIVES
Simply put, your drive regardless of the media holding the 1's and 0's can only process ONE R or W request at a time, and like the DMV, you can and do end up with a very long line (queue) because of this. Performance slows, and nobody likes this. By the nature of their inherently PARALLEL internal operation combined with very high bandwidth per die (and provided it's equipped with a competent, or if Samsung then excellent, controller... An area where the horrible OCZ failed spectacularly and repeatedly, and why the "me too" companies like Corsair that drive on imitation without innovation, slapping their logo on other companies' products, are the scourge of progress because the ONLY thing driving product "development" is money, instead of innovative product design and money), SSD's are able to very quickly get through the queued operations, but the longer the line gets, the less responsive the drive feels, as it prioritizes different requests constantly.
The above is the reason why SSD's provide such incredible improvement in user experience for the OS, which consists of MANY tiny files and very rarely doesn't necessitate the term "random".
The shiny big numbers used to advertise SSD's, the "550/525" etc, refer to sequential throughput which is basically an "ideal" situation, one dealing with very large files. The benefits of the seemingly impressive seq speeds are quite low... SOME programs and games load more quickly as a result, but the difference is far from groundbreaking; I measured load times of BF3 on both SSD and the incredibly awesome and budget friendly top hard drive ie the WD10EZEX, and the averagediffe rence between the two was 3.9sec... Dropping from the low 40's to the very high 30's; not a big difference.
One of the big reasons why having the OS on an HDD feels so slow is because most people have one drive, MAYBE 2, in the syste. The result is a drive that is never able to keep up with the myriad and relentless requests from the many programs running, on top of the OS.
While obviously not as much of an improvement as an SSD, simply having a dedicated hard drive for nothing but the OS and another for everything else provides a great boost in system responsiveness, and if you short-stroke the OS drive and/or utilize a faster spinning drive, you further improve performance
I have my 2nd HTPC running on old repurposed hardware, including the (two, RAID0) OS Drive's, a pair of 150GB VelociRaptors short-stroked to 30GB each for a total of 60gigs, all of which is at the absolute edge (fastest location) of the platter(s). It is pushing a max of 382MB/s, average of 379, and min of 374, across the partition, and access time is avg 2.9ms.
Needless to say, it is not as fast as an SSD. I have used an SSD as the OS drive, and apart from the sped up boot process, I felt absolutely no difference in use. The OS has not once caused this heavily used HTPC to slow down. (i7 920 4Ghz, X58 Deluxe, 24GB DDR3-2000 8-9-8, EVGA GTX570 1.25GB, Intel 2x10+4x1GbEth PCIe HW NIC, LSI 9265 with expander/BBU, 2x150GB VR, 2x 5x2TB RAID6 7200.14's, 2x 4x4TB RAID10 HGST Ultrastars)
The bottom line is this: the more drives you have, so long as you spread the data across them intelligently, the faster and less of a bottleneck your storage subsystem will become.
Intelligent use of techniques like short-stroking (ie using the outermost 20-35GB of a long-term storage drive, whose main data is almost never accessed and/or consists of simple to read highly sequential files such as movies, for the system TEMP/TMP directory and the page file as well as a very write intensive directory like AppData; I have such a setup, across two 25GB short-stroked WD10EZEX in RAID0 with the rest of the space holding my pictures, downloaded program installers, and the like, and I have not once perceived a single solitary difference in performance), as well as utilizing RAID appropriately, further enhance and improve the performance of the system and if done well, have miniscule to no trade-offs.
Here is an example of heavily parallelized storage that has resulted in (no objective measurement, but an experienced subjective measurement is...) tremendous improvements in performance, which becomes more and more noticeable the more programs you have open, provided you accurately positioned such programs across the drives.
(quickly: if you spend 75pct of your time with Firefox running, MS Office program/s open, Torrent Client downloading in background, and your media player of choice playing music or outputting video to a second display, while the other 25pct is gaming, you will find that having C: for OS and maybe Office, D: for games and media programs and utilities, E: for music and pictures storage, F: as a movies/shows drive, G: as the first 20-50GB in a partition for temp and such while H: is a second partition but of 2x or more the size for use as a download temporary directory while I: is the other 0.8-3.8TB for "deep freeze" storage meaning files that are never accessed but need stored... Even with "only" C being an SSD, and the rest being 500GB-to-4TB disks, you will have a very fast and incredibly responsive system even when multitasking heavy duty. You canfu rther improve aspects by utilizing RAID for the redundunduncy and/or the speed benefits, or you can get a nice 2 to 4 Bay "destop" DAS without but preferably with its own integrated RAID controller and even a decent amount of cache, and which connects to the system via eSATA6Gbps or Thunderbolt or whatever the fastest interface you have is; this provides you with a LOT of "runoff space" either from the get or built up over time, and I find them, in particular the class-leading Areca boxes, to be the ultimate way to store media: 2-12x drives of 1-4TB in a number of RAID1/10/6 arrays providing a LOT of storage space, a LOT of speed potential esp with the Areca units as they have full HW RAID, and the best thing is that it is completely transferable between PC's! You don't need 3+ XL drives holding movies and such on a gaming PC when you have even a 1-2yr old laptop or older still desktop that is 100percent HTPC ready, and instead you can use the external drive setup to simply plug into the HTPC for use, immensely awesome when you are using a laptop for the duty, but you can quickly move it to the gaming PC, the one connected directly to the high end router and getting every possible drop of the 25 or 50Mbps download speed, and rather than trying to download a large collection of films/show episodes with a barely present Wi-Fi signal from 3 walls and 2 floors away which is barely able to max at 500Kbps thus spending weeks where the computer is sluggish and playback is choppy, you can download the files directly to the proper drives and folders, taking full advantage of the 6ft CAT6E connection with the router (or 2 cables, teamed/multiplexed) to get a steady download rate of 4-6MB/s actual, start the process before bed and begin your Super Radical 80's Movie Marathon before breakfast the next day, with the only thing you need to do being unplug, walk, plug in, turn on, leaving you free to go get frisky with hairspray and acid washed jeans).
In your computer storage subsystem, sharing is not caring... Selfish drives are happy drives.