The modern Hi-Iso cameras are incredible at focusing and for natural light. If you never have a need for that, don't talk about it. I've pushed my Nikon D4 into taking pictures in light so dim that I couldn't see.
Ellis, all true. Try cropping out 35% of your pixels and enlarging and see what happens. There simply is no replacement for more MPs, no matter how nice a lower MP file is.
If one needs the speed of the 1DX, then it's worth it. If you need to shoot over 1600ISO, then it's worth it. Barring those two specifics, I don't think "upgrading" from the 1DSMKIII is going t be very convincing to most people. You can do a lot of high quality noise reduction in software these days, but you can never add MPs to a file. And then there is the 36+ MP Nikon. I can't understand what canon is ding expect pushing pros to Nikon, where there are more options.