Because you get stupid stuff like the Michael Jackson thread, and a thread the other day where someone replied to a 3 year old thread with something completely off topic. If it adds to the conversation then no, it's not a big deal. But more often than not people end up replying to the old stuff in the thread rather than the new post. That can be especially annoying when it is outdated/superceeded/irrelevant material.
Anyway, I didn't start this, but I'm glad I'm not the only one annoyed by it.