I don't see the problem with goodbye threads, especially when the person quitting tells about their experience playing the game. We do after all, come here to read about each other's experiences in addition to sharing information about the game right? Like it was already mentioned, there are far more useless threads than someone posting a farewell to friends that frequent the forum. It may not serve you any purpose to hear about someone quitting, but often there is at least one person who gets something out of it.
I do however think that there should be a forum for returning and quitting players though. To me the "Good Times" thread is more of a place to post about current accomplishments within the game, and while a farewell thread may include accomplishments within the player's gaming experience, its not always the case and sometimes its just a plain and simple "goodbye, thank you".