Ah - another insightful rant from Clueless Kenny, the uber math whiz.
I'm quite curious how f/1.0 can be 1/2 to 2/3 stop faster than f/1.2... what causes it to vary? Phase of the moon?
Comparing the entrance pupils, the ratio must be (1.2)^2 = 1.44. In terms of stops that's 0.52. Ok, so it's half a stop faster than f/1.2 ....
But it's only half a stop better if the camera detects the light. Other's have commented (and my tests have confirmed) that my 1D4 and 5D2 cannot actually see light from a cone that's faster than f/1.6. This does vary from lens to lens - some modern lenses are telecentric so that all the light gets to the photodiode. However, the 50/1.0 comes from the film era and is not telecentric. Ergo, NO fast aperture, NO narrow DoF and NO benefit.
Well, I suppose your wallet would be lighter, even if your camera bag wasn't.