Those unity stats are interesting. They do represent more of a 'casual' market than a 'hardcore' market, but 20% of their users maxing out at SM2.0 still surprised me.

I guess a full inclusive summary of those stats is, if targeting unity's customer base, the feature-level/shader-model you target will let you reach these percentages:
SM5.0 -- 5% -- DX11 feature level 11
SM4.1 -- 20.8% -- DX11 feature level 10.1
SM4.0 -- 51.2% -- DX11 feature level 10.0
SM3.0 -- 66.1% -- DX9 w/ SM3 caps (for some strange reason, there is no DX11 feature level for SM3!! WTF?)
SM2.0 -- 89.3% -- DX9 or DX11 feature level 9.x (N.B. actually 53.2% if using DX11 here, due to the OS restriction)

N.B. Every ("A grade") PC game that I've bought for the past 5-ish years has required SM3 (this is because the 360/PS3 are basically SM3), and newer PC games are starting to require SM4.
It seems for casual games with low hardware requirements though, supporting DX9+SM2 might still be a good idea if you want anyone to be able to play it.

Personally, I'm still supporting DX9+SM3 as my minimum, because I want wide OS/hardware support, but I find SM2 is such a pain to program good shaders on.
I'll add a GL2+SM3 path when porting to Mac/linux, add DX11-feature-level-10 code-path when I get time, and eventually add a full dx11/GL4 path one day...

In my eyes, DX11-feature-level-9 is completely useless -- It lets you access the SM2 hardware of 89% of the customer base, but restricts you to the OS that's only installed on 53.2% of the customer base... IMO if you want to support that old hardware, you should just use DX9.