For me Community Goals were always more about the COMMUNITY aspect rather than the goal. It's the one place in this "MMO" where the multiplayer feels "massive" rather than a single-player game with some very random encounters with hollow squares on the radar. I'd argue this is even more necessary in Legacy, which people say has way less players than Live.
I don't care why the people at a CG are there (trophies, rewards, PvP, etc), I care that they are there.
EDIT
That said, I don't like AX CGs because I have zero interest in AX..