Well they either "leave" and come back, or leave because they got banned etc usually. There is no need of spamming and saying "I am leaving PokeBeach" when you are just gonna change your mind and come back the next day anyways. I mean if they have been going to beach for a long time, why would they just up and leave unless it was a serious, serious thing.
And that usually involves bans because nothing ever super serious that would make somebody want to never come back here again after going here for years. So I don't think it is a big deal, welcome threads are ok, because who is gonna leave the same day they join? So goodbye threads seem pretty pointless, a lot more pointless than welcome threads.