I never personally saw the point of "leaving" a forum.
It takes more effort to leave than it does to...not leave and just not post often if it's not what you feel like doing.
If I feel like KMC is getting stale, I don't say I'm leaving, there's no point. Just don't post, come back when there's something to post about. This is the forum I've been on longest, and I didn't even think I'd be here this long.
-AC