It's really not as bad as some may think. Yes, there are times when certain threads should be left alone (like single questions that should've been asked in a FAQ thread), but there are other times when it is acceptable to bump a thread. It simply depends on what it is.
Sometimes when one feels like sharing something, and they see an old thread with no new replies, they may decide to post to that thread instead of making a new one. I think that's actually better than making a duplicate as that only causes clutter. As long as a bump contains content relevant to the discussion, then that should be okay.
Bumping a thread just to grab attention, on the other hand, is not as good. If a week old thread is bumped multiple times and all the posts say along the lines are "y is no 1 respondin??? pls post", then we can talk potential spamming. Other times, I've seen 7+ year old threads on other forums that were bumped, and the offender only bumped it to ask a question. Like yeah, the OP is totally going to come back and respond to your question after seven years of inactivity.
Some forums discourage bumping entirely (like some tech help forums), and I sort of understand why. In my opinion, however, certain bumps can be highly justified.