Here's a few reasons why, despite loving the premise behind mITX, the format just isn't for me. And just to spice up the mix, I'm going to add comments regarding the two computers I'm building in the near future: one being a gaming pc, the other a digital audio workstation.
- No expansion options. One PCIe slot forces you to use external USB audio cards if you want higher quality than onboard audio, thereby writing off the whole sizing benefits in the first place. And yes, for the record, I consider an audio card a requirement even at this day and age of respectably good quality for onboard audio. As for the DAW, a single PCIe slot makes it a complete PITA to add in any kind of hardware acceleration (Avid ProTools|HDX cards, Universal Audio UAD2 cards, etc) - or should I say IMPOSSIBLE to add in. Others have already commented on the multiple GPU bit.
- 16GB maximum RAM. No offense, but even on a gaming rig this reduces your options, since new games are now recommending 16GB of RAM in a system. You more than likely can't even think about using a RAMdisk, as an example. For a DAW, I sure hope you aren't planning on doing anything memory intensive (most high quality plugins for DAWs can easily chew through 1.5-4GB of RAM per instance!)
- Cramped quarters. mITX cases are TINY, unless we're talking about a Case Labs S3 here, but if we're trying to use the whole "it saves space on your desk!" argument there's no point bringing Case Labs into this argument. I went with a full tower ATX case for a reason, and my HAF 932 was a dream to work in for setting things up on my previous build. I don't want to deal with the issues smaller cases involve, as my hands are huge.
- Expandability, repeated. The Maximus VI Impact can only have a total of 6x USB 3.0 and 6x USB 2.0 ports, compared to the 8x USB 3.0 and 8x USB 2.0 ports with the Maximus VI Formula. For gaming, I suppose that many ports could work. For audio/DAW use? My current setup already uses up six ports alone (mechanical keyboard, mouse, Akai MAX 49 keyboard controller, NI Maschine mkII, and NI Kontrol Z2 mixer), and my planned expansions over the next year will eat up another 7 ports easily (NI Maschine Studio, Ableton Push, some sort of 8x8 I/O MIDI rackmount, Novation UltraNova, one free port for updating my Arturia MiniBrute or any Elektron gear I pick up, a second mechanical keyboard for the live PA (or I suppose Traktor Scrach Pro 2) "operator's console", a trackball mouse with said operator's console, etc etc).. This would leave me another 3 ports and make it so I wouldn't have to use any hubs (which can cause issues when dealing with audio stuff, as recording/performing gear is notoriously picky about those kinds of things).
- Hard Drives. I'll second what everyone else has said, but add this part in. Recording audio is notorious for it's consumption of hard drive capacity, especially when it is at a higher bit rate. And then you add in redundancies so the death of a hard drive doesn't set you back hours/days/weeks of work. And for gaming? Those humble bundles keep chewing away at my hard drive's capacity. Sure, one could argue for only installing the games you want to play then and there, but I totally hate waiting for a game to download, install, and then do first time setup. Additionally, I'd rather just download the game once and be done with the bandwidth hit - not everyone has limitless bandwidth allocations.
- SATA ports. mITX only has four, and if you want/need an optical drive that drops you down to three available. I pity the person who tries to make a high capacity HTPC with just three drives for storing movies/tv shows/anime (or dare I say two, if one goes with a SSD boot drive)
- Choice: There are a LOT more board options in ATX or mATX than there are in mITX. WAY more. Choice of what Network controller is on the board, extra raid features, extra/better onboard audio, PLX chipsets for multiple GPUs, you name it. I can also have my pick of the litter when it comes to colour choices as well!
- Socket compatibility. One might not be able to find a board compatible with a processor you want. Want a socket 2011 mITX board? And you want it with high end features too? More than likely a no dice no love situation for a hexcore setup.
- Watercooling compatibility. Others have already commented on this. This is an enthusiast website, so I'm going to venture a guess most of the members like their "optional frills," not to mention tweakability, even if it is just to make things quieter.
- Easier to work in environments. Larger cases are naturally easier for a number of us to work in, and it's also a lovely benefit to be able to get BOTH hands in to secure something when you're working away.
- TIdyness. Arguably, it's easier to keep wiring tidy in a larger case, because you actually have places to hide wiring and whatnot.
- Bay Expansions: Larger cases can naturally fit more things in them, be it fan controllers, LCD displays, optical drives, card readers, hot swap hard drive bays, bay mount reservoirs for watercooling, or even just plain old ventilation fans. Whatever you can think of putting into a bay (even a cupholder!!!) can be put into one if you've the room to do such.
- HDD compatibility. Not all small form factor cases allow full sized 3.5" hard drives, which limits you to 1TB HDDs at less efficient speeds (most laptop drives run at 5400rpm)
- Living room domination. This one's more of a silly thing than serious, but it has to be said anyways. Large towers look awesome in a tech-centered living room, especially when you have 3-4 of them! Spousal acceptance factor may or may not play in on this one, but I'm good since my spouse loves full size towers and won't have it any other way
- Headphone storage location. My HAF 932 has a convenient spot to put my headphones, and that's right on top, where the whole rubber cover thingy is. It's great to have them close, yet still out of the way.
- Cost versus value. Due to the miniaturization, you're typically paying extra versus mATX and ATX, and getting less features as the size constraints force developers to pick and choose what they can actually put on the motherboard. A similarly priced ATX motherboard will pretty much always have more extras than a mITX board.
Am I saying that everyone should have a Case Labs STH10, and an HPTX motherboard? Hell no. They were designed for excessive setups, and those who've made those kinds of purchases seem quite happy with them. But I'm pretty certain that, at least for myself, the larger cases are quite lovely to work with, I enjoy showing off my gear, and it's a hell of a lot easier to fit what I want as well as need into. As neat as the concept of a shoebox PC is, most of my needs (specifically centered around expandability, whether required or just a nice thought) are not met by the mITX platform.