Is anyone here actually shooting with these things or just talking? If you mean that it's impossible to get a great key with the 7d then... well... you're completely wrong.
It is possible to get beautiful shots out of a 7D. It is also possible to get horribly ruined shots out of a 7D, and greenscreen is one of the places that is more likely to show such ruination. The purple/green aliasing can (I said CAN, not ALWAYS DEFINITELY WILL) cause problems ranging from mild to severe when working with greenscreen.
Oh, yea, I didn't think of that! That would suck, all it takes is a high frequency surface and you'd get some traces of green that would blast holes through the subject.
Of course the 4:2:0 color sampling would be an other disadvantage, a horrible one for things like hair at full 1080p.
