The thing is that you do have to crop when shooting with primes, at least for weddings which is what I do, you are shooting moments and can't always have an exact composition in the way that you can with a zoom. On the other hand there is so much resolution and sharpness shooting a 5D and primes that as long as all the necessary detail has been resolved, you just don't need any more and you have plenty room to crop before you reach that buffer in most cases. One thing that I've learned is that shooting with top primes will bring a huge boost in resolution compared to even top L zooms. Shooting group shots with my cheap 50mm f1.4 at f5.6 absolutely creams my 24-70L for detail and contrast (and I've owned 3 copies of the zoom!) and my 85mm 1.8 is much better than that even.
It does of course boil down to your needs, if you are shooting in a controllable enviroment then you shouldn't need to crop, it's not a problem to get everything right in the viewfinder. If you're shooting in an uncontrollable situation then there is little that needs that level of resolved detail.
I do think that 12-13 megapixels is the sweet spot for a 35mm form factor DSLR, big pixels with great DR, easy to work with files and providing good glass, enough resolution to satisfy the vast majority of needs.