You're right, I think - after a certain thread length is reached, it's really only the usual suspects present any more, as far as I can tell. And what's the point of that, really? Catharsis?
Now, it occurs to me that there's a grant application hiding in there somewhere - we need to sit down and calculate what that length is, so we know when to close and lock threads. If we can find where the "crevo event horizon" is, we can keep these threads from becoming marathon flame-fests. ;)
LOL, my instincts (evolutionary-based, of course) would suggest that when certain persons visit the thread, that's the time to close the thread. However, I'll be good and not say that :)