In which situation would this be useful? I don't think people AFK by accident, they do that to get their gear repaired or melded or to bazaar or because they want to follow the LS conversation or for whatever reason. I don't think anyone really forgets to log out, they just don't want to. Only case where I could maybe see this being useful is when someone falls asleep while playing, but if that happens too often, that person would need some other kind of help than auto-logout feature.