First of all the percentage of people who actually overclock their CPU is very small, most likely in the single digit percentages. With well over 90% of people running their CPU at stock clocks it is important that AMD and Intel offer CPU's with different clock speeds and different features. It is called tiering.
Basically they are all the same CPU but some people are willing to pay $1000 for a CPU and some people are willing to pay only $100. So what they do is design one CPU, since that is much cheaper than designing multiple desktop CPU's, and then set them at different clock speeds and cut features as they get cheaper. That way the people willing to pay the extra money will more often than not pay for the higher end parts. The people who only want to spend $100 will buy a Celeron.
You can see that with the latest generation Intel has decided to make overclocking one of those tiered features, requiring you to not only buy a higher end model, but buy a certain chipset too, their chipset by the way. It is a shameless money grab that doesn't make a lot of sense in my opinion, as overclockers are a small but vocal set of their customers. They are the ones that recommend to their family and friends what kind of computer to get.
Anyway, with Sandy Bridge the days of buying one of the cheapest model CPU's and overclocking it to match the higher end ones is gone, at least with Intel chips, at least this generation. AMD is dominating the value segment anyway, the under $200 segment.
Hopefully Bulldozer will bring AMD parity, which it sounds like it might, although maybe not clock for clock(sigh), so that Intel can't play these games.