Well, a few reasons. First, any project of a certain size needs bug tracking. It helps you, and it helps your users. Second, if you've already got a tracker (Garage, elsewhere) you should consider moving Bugzilla since it's centralized (most users reporting bugs go to b.m.o to do so) and a vastly better tracking platform than GForge. Third, b.m.o comes with its own support staff of administrators and triagers who will be able to pick up some of your bug maintenance workload.