At f/1.2 focus accuracy is only going to be so accurate ...
That´s the main point. If you are looking to get the f1.2 lenses, you should have the best available AF, to cope with the very shallow DOF. And if you can´t afford the 1DX, then 5DIII is next in line.
that is why i chose 5D3 over the 6D. Plus the ergonomics. ok the 6D is probably a stop better at high ISO, but i really needed the dual card and the AF system of 5D. I've searched it a lot, even here in canonforum before i make a decision on which camera should i buy.
Anyway, i tried the 85 1.8 at 1.8 and it focus accurately. So i'm thinking the f/1.2 would make a perfect combo with the AF of 5D3.
and as much as i read there will be no successors to these lenses any time soon, so i'm going for it.