Not to be a jerk but even if the 5D Mark III has better ISO performance, I don't know any one that WANTS to shoot at high ISOs. Thats something that you do when you HAVE to. You want the best image that you can get, which today means using the lowest possible ISO. In my experience, most people try to stay under 1600. I might be missing something but 2 more stops is nice but not a "game changer" for most people that I know. Am I missing something?
As far as I can see, if new camera bodies have better noise control at high ISOs, it also translates to better noise control at lower ISOs. For example, if 5D2's max tolerable noise control is at ISO3200, 5D3's might be at 12800. This may also bring more pleasant image quality to 5D3's images at ISO3200 as compared to the 5D2's.
It is not a game changer, but it's a good to have. What people are really looking forward to is a refined AF system. THAT, is a game changer and a head turner.
For some, higher ISO capability means being able to get a shot, or not. You sre right, no one wants to use high ISO for the sake of high ISO, but when light is low, flash is not allowed, and the subjects are moving, you cannot have too high of a ISO capability. Just last week, at our school play (I shoot images for the plays), there were lots of scenes with virtually no lights and dancers moving quickly. ISO 6400 and F:/1.4 are no where near fast enough.
I had to throw out 90% of the action images due to motion blur, and those that remained were still a little blurred. If I had IS) 51, 200, I'd be using it for this type of event. I also took my 7D to a night time rodeo. Really fast action and very dim light. It was nearly impossible, once again to freeze the motion.
If you have a high ISO capability, it opens up possibilities that were just impossible with my 40D.
This photo was taken with my 5D MK II and 135mmL at ISO 6400, f:/2 and 1/125 sec. The actor was not moving and the spotlight was dim, but it turned out.
This is a motion blurred image. I needed a faster shutter speed, but could not get it without having a couple more stops of High ISO.