When we add code to Tremulous we're essentially making a commitment to debug and maintain it indefinitely. This quickly becomes impossible if we include a bunch of ugly hacks. People submit patches that on the surface work at the moment and then they ride off into the sunset, but if those patches are messy, we, not them, would be the ones to deal with that mess indefinitely.
Obviously I used to be on the other end of this, making poorly coded features with hacky workarounds, and including them in unofficial releases that people used and liked, but it did become a buggy impossible to maintain mess, and it was only possible in the first place because trunk is kept to a higher, cleaner standard.