When people say photos are soft, they typically mean that focus was not achieved. That might be because the camera and lens are not properly matched, it picked the wrong thing to focus on or something else but...
Focus not being achieved is different to blur as a result of either the camera or the subject. IS only helps deal with blur because of camera motion.
At 50mm, you should be using the 1.2L or 1.4, which would give you a much faster shutter speed than the 2.8 for the same light.
Yeah I was shooting with the 50mm f1.4 but it's not always practical to shoot at f1.4 as I often need more depth of field than that. There are only certain shoots you can shoot wide open like that.
My shots where focused correctly... the slight softness was due to motion blur.
Motion blur of the camera or the subject?
I have shot with the 17-55mm f2.8 IS on a crop camera and gotten crisp shots at 1/20th of second.
So? I've taken sharp shots with the 50/1.4 at even slower shutter speeds of static subjects, hand held.
Anyway, you seem to have convinced yourself that you need IS on whatever lens you use, so good luck.