I would probably not use the 1D X for weddings, personally, if I also owned a 5D Mark III. I don't know about anyone else, and sorry if I'm pushing too far here, but in my photos I will certainly notice the 22 vs 18mp. It matters, trust me. You can get good enough lenses and the ISO performance on that camera is superb. That's why I think the 1D Mark IV causes some problems for some people. I like the absolute most details in my photos because I print large and I crop for super detail. 22mp will benefit me here. I'm not at all suggesting that 36mp is necessary (D800) but I can notice 22mp vs 18mp vs 16mp.
However, the 1DX vs. 1D Mk 4 for photojournalism and sports will be unmatched. You can be assured I will buy a 1DX after it's been out a few months, if I can get one, so I'm obviously all for the 1D X. Looking at a lot of my sports photos I was wishing the 1D4 had been higher mp, because in some cases I had to crop in too far, even with a 300mm lens that I could see the detail loss. I agree with Mark that the 1D4's auto focus tracking could be improved, and I believe the 1DX does just that. Let's see how it goes!!