Clean Install
It just "feels" better to do a clean install; that's probably the biggest reason people do it. I don't expect it to make a perfectly functioning system work any better than if you would have only upgraded. My policy is to always preform a fresh install after every OS X release. Primarily because I like things neat, and it's good way to clean out all the meaningless folders, files, settings, plugins, and system changes left behind by old programs. So for me it's mostly a "state of mind". Plus I'd rather rebuild my system freshly from backup data than trust automated transfers.