Funny you should ask, I don't really know. Habit probably. Every time I think of leaving, one or two new posters become active, who's opinions I enjoy reading. Unlike you,
Rojo, I do think the intelligence level here is generally pretty decent, although there are a number of posters like those you mention.
I do sometimes think of promising favours to the admins or sending a check, if only they would permaban one or two names. There is no one really bad who truly deserves a ban, but some peeps are just so irritating, after years of repeating the same things, I know what some posters' hang-up responses will be before I've opened the topic. I admire the self control of the admins.

Having said that, no one's ever annoyed me enough for me to place them on ignore.
I guess it's pretty enjoyable to have a place to go on the internet where you know people well, and they in turn know you, and you share similar interests, a place that allows you to get comfortable and put your feet up, so to speak. The place is like a pair of comfortable old boots for me.