I guess posting such a topic is a typical psychological pattern for people in a certain stadium of the game. Though I don't know which. It's probably when they get too used to the game or feel disappointed by a patch (=nerf), but they lack some kind of alternative. They feel like they don't want any more, but they can't imagine being without cRPG as well. That's why they post such topics, perhaps to give the others a chance to convince them to stay, or perhaps to support the own decision in the way of "now I made my decision official, now I have to stick to it". That the last point doesn't really work that often shows how unique cRPG is, because there is no other game which is similar to it and thus could replace it.
On the other hand, someone who is REALLY fed up with the game, doesn't feel the urge to tell the community anything, because he doesn't feel any connection to it. Except of a few friends/clanmates, but then the player will tell them personally.
We should actually use every "I quit" topic to ask the author about his feelings about cRPG, and help him find his REAL decision, instead of just posting "gonna miss u" "kthxbye" and "who are you?". If we want.