While it is true you are more limited with your AF points on the 6D, keep in mind you can focus and crop instead of recompose.
Certainly, but I'd consider it a a workaround in the extreme.
More importantly, the 6D allows for easy switch to Eg-S focus screen unlike the 5D3 - so IMO 6D actually has a significant advantage for manually focusing the f/1.2 lenses and seeing their true DOF in the viewfinder.
Yes, the focusing screen can be installed (I have the Eg-S screen) but I'm specifically commenting on the AF which doesn't work too well with very large aperture lenses. Comparing the 5D3 AF with the 6D MF with Eg-S screen is like comparing apples to oranges. The 5D3 outer AF points will lock on with superb precision and you would have already taken the shot and moved on to another in the same time you would be focusing manually with the 6D.
One could argue you are looking for trouble in general if you try to autofocus at f/1.2 all the time no matter what camera you use.
All EOS cameras AF with the lenses wide open, so a f/1.2 lens will AF wide open at f/1.2 no matter what settings you use to shoot. The cross type outer AF points work extremely well on the 5D3. See below photo taken with a 5D3 with 85L II - 100% crop of the AF area also provided - I have been using this combo for quite a while now and am happy with the results.
Not saying that 6D is a bad camera, but the whinging that takes place over the AF happens deservedly so.
Edit: I'm not sure how to post the 100% crop. The 100% crop image that I can view here on CR is larger than what I see in LR with a 1:1 view - any tips on how to post the exact image?