Why would you want to avoid it? There is at least in theory one good reason. IP discards the whole datagram if one of its fragments is lost. Assume you send a 4 kilobyte datagram with a MTU of 1280, respectively 4 fragments. If you do your own fragmentation, those "fragments" are complete datagrams. If one is lost, you still get the other three. Relying on IP fragmentation means that if one is lost, you lose all four.
Now, I haven't looked in detail at the fragmentation code, but I was assuming that losing a fragment would invalidate the entire packet anyway - i.e. one lost fragment means you lost all, just like with IP.
Is there no merit to my conjecture that it can be worthwhile to pre-fragment packets to avoid any reassembly at network boundaries and minimize MTU-related issues?
Of course, it might simply be that they wanted to allow packets that exceeded UDP max packet size.