The 5d2's ability in low light is much lesser than the 6d. If you are never in ISO 6400 and up situations there is no reason to choose a 6d over a 5d2, but if you shoot in low light the 6d is vastly improved over the 5d2.
I agree and surely these other improvements must carry some weight?
full support for rt flashes
dual afma for zooms
flexible min/max auto-iso
min shutter speed setting
As has been said on this forum (correctly) a camera is so much more that a sensor. I really believe that if Canon had just put 11 good cross AF points in the 6D there wouldn’t be any arguments about 6D over 5D2 and that wouldn’t have taken anything away from the 5D3.