Originally posted by SOldBear The only thing I can think of is flash exposure. It doesn't matter in p-TTL mode, but if you're using manual or auto flash mode, not knowing the exact aperture can be a problem.
To add to that, just a constant exposure in general so your settings aren't randomly changing on the fly based on zooming a lens in/out. Also, fixed aperture lenses tend to be faster than the variable aperture equivalents (at least F4, or F2.8) which allows for faster shooting in available light, along with shallower depth of field (at least with F2.8 lenses).