I was thinking about this and my conclusions so far are:
- although supporting SM2 backwards compatibility is nice practice, I've never ever had to use it before
(other then testing if backwards compatibility/ rendering with SM2 works OK in my engine/ with my shaders)
- I believe SM3 is basically standard on all GPU's since around 2006
- currently backwards supporting SM2 brings extra work, less features, extra maintenance when doing changes etc.
What do you think?