To me, zooming in or out with your feet is kind of beside the point. If it's just about what I want covered in the frame, yeah, sometimes that works. But to me it's more about the geometries created by a wide angles, how lines and angles come together, when its morphs from angles into distortion, etc. I find 24mm equivalent lenses to really pull all angles and lines to the center pretty radically, but stopping short of getting out of hand. 28 looks just about normal to me, 35 verges on realism, and a neutral 50 just bores the hell out of me. A UWA like 14-18 is pretty bizarre and i can only make it work in limited circumstances and a fisheye has a whole other thing going on. Nobody shoots a fisheye for more coverage - you shoot a fisheye for the fishi-ness effect. Whether a 21 will work for me remains to be seen. Buts my love of wide angles has very little to do with what I can fit into the frame and everything to do with how it pulls the elements together and the kinds of compositions they encourage. And I guarantee a 21 will be very different than a 28.
-Ray