For the guys/gals that use just primes with cameras such as the 1DsIII and the 5D3, what are your reasons behind that (sharper, faster, etc)?
I have a 5DII and have all priems (35mm f/1.4, Sigma 85mm f/1.4, Canon 135mm f/2, Canon 50mm f/1.4) The main reason I go for primes is that they're faster-- more options with depth of field. I started out with a crop camera with 15-85mm and the 50mm f/1.4. The zoom just didn't work for me and I grew to like having the faster aperture. At the tele end, the 15-85mm has equivalent dof to f/9 on a full frame. On a full frame, an f/2.8 zoom would probably be good enough for me (both for shallow dof and in that I can crank the ISO higher than on APS-C) but I've gotten used to shooting with primes.
I don't really care for having the sharpest lens out there -- I think sharpness is overrated.