Aside from getting a bit more in-camera 'reach' from lenses, the other benefit of crop modes is that you can output a significantly smaller file sizes, which I expect is a big bonus for sports togs who want to get their files transmitted quickly. Plus, in some instances, the fps increases when the crop mode is activated. Crop modes can have an effect on the number of AF points available though.
I loved the crop mode on my D2x, really useful.