I'm tired of teaming up with deadbeats and cooks in the public games and getting my ass handed to me. I'd rather team up with newer players with some skill and (more importantly) reliability - and I've played with some, don't get me wrong, but the deadbeats and loners will always drag you down.
Another advantage to joining a clan will be playing with the same people regularly, enjoying some camaraderie, and learning from their styles of play.
If you know of a good 'rookie' clan - one that values participation and good communication, drop me a line. Or, if you find yourself in the same boat, let me know; maybe we can start one!