I run a small forum and occasionally somebody will post a new question on an old thread that's about the same topic. My two theories are that either they searched, ended up on that thread, then just replied because it was in front of them, or that they hope the original participants are going to be notified of their reply so it'll get more attention than a new thread. Unfortunately, on some forums that's actually true and encourages this mess-making behavior.

I don't see the problem with "omg me too!" on an old thread as long as "omg me too!" is acceptable on a new one.