For the sports shooters that want faster than 1/250 shutter speed as minimum, what is your application?
With the feature set of the 5DII, I would always shoot sports in shutter priority (Tv) (which can be fixed to as fast as 1/8000). Now with the flexibility of AutoISO in manual mode (M), I also have ability to fix the shutter speed and the aperture while leaving AutoISO to compensate for changes in lighting.
Now with the 5DIII, I would use the minimum shutter speed in Aperture mode, which I would use for events not sports. The minimum shutter speed would safely ensure that I don't have blurry pictures as my subjects will not not moving fast.
This sums it up well, and if I may add..
Why would you need such a high "minimum" shutter speed? A minimum setting is just that, setting a LOW point. This is to prevent those blurry pics while in Av mode.
Why would anyone shoot sports or any other action shots in Av mode? Most would use Tv and set a good shutter speed to catch action with the option of letting the camera auto adjust ISO.
But wait, now we have changing lighting conditions and are afraid that the auto ISO won't be enough...Since I may not be thinking about light, Do I actually have to check my exposure and adjust my high dollar camera, which has infinitesimal adjustments in it, turning a knob here and dial there to get a good exposure?? NOOOOO! Just put it in the green box! Yes, now the camera will do everything and I don't have to think about what it's doing, and since you can now buy a cheaper camera that comes with a green box you will save money. Bonus! it will even come with a flash...
I also like that auto ISO has been included in M mode on the mkIII. And FYI on the mkII the ISO is not "locked" at 400, it is automatically set to 400. You can adjust it to whatever you want.