I'm with Steven on this one. Squats have always been my favorite lift, and I've never felt the need for a weight belt.
My line of thinking has been, as I squat, I'm not only strengthening my legs, but I'm also strengthening my back, abs, core, etc.
Using a weight belt forces those core muscles to work less, and they won't get stronger in parallel with my legs. Besides, if I can work extra groups of muscles that are important along with other lifts, why wouldn't I do it? Multi-tasking is a great thing