Short answer, is that no matter what af point you use, it will focus in less light (-2EV) than the 5D2 (-0.5EV). Also, the centre point of the 1DX/5D3 is a f/2.8 cross (or is it a double-cross?) point, which will be a lot more accurate than the 5D2's centre point.

Even if it's the only point you'll ever use, it's a lot better than the 5D2's centre point. And trust me, once you get used to more AF points, you'll wonder how you got anlong without them (I couldn't use less than 19 ever again, sigh...)