because 50mm is a everyday use lens. sometime one might want whole pic to be sharp, esp if u using it to shoot group photo
I do shoot a lot of group photos (can't help it) and a fair bit with the 30/1.4 on a DX body. From my memory 6R, S8R no issue. I have never recalled any serious issue with other photographers earning a living from the 30 or 50mm Sigmas stopped down. Like I said, I can't help it, and ever even risked 12-24 before with group pix (bad idea, not very sharp). Now I use other techniques...
Of course I am not saying that you are wrong (perhaps you shoot professionally as well, at a much higher level than me). Or you are a hobbyist going after the highest quality available with MF digital backs with 80K worth of equipment and this is just a play play "bang around" lens.
Now here's the 1D MarkIII, and vs the 50 1.2L $$$ glass. If I am not shortsighted or something wrong with my eyes, the Sigma is better than the Canon L.

Bokeh I'll let the end-user decide, perhaps the L would edge the Sigma out by a fair bit and that might be what matters ultimately for this with the moola.
http://www.the-digital-picture.com/...p=403&CameraComp=453&SampleComp=0&FLI=0&API=5
(in fact the corner sharpness/contrast for the Sigma at f1.4 is better than the Canon L at f2.0)
PS. Seriously, the 2 edges (left / right) are more important than the extreme corners. And are often better than the extreme corners.
Sometimes I just wished Sigma do up the 30/1.4 as nice as the 50/1.4.