There are actually many online forums that prohibit the practice of bumping threads, and at least one for which it will get you banned. The philosophy behind bumping itself is a little suspect IMHO... It seems to me that the bump-er is saying that "My favored thread is more important than all these other threads that people are replying to more readily."

It seems to me that the software for these forums is designed so that the most recently-replied-to threads are at the top, and that's deliberate.
By the same token, it is interesting to note that bumping has the reverse affect on folks who surf these forums as I do: I have bookmarks in my Favorites for each of the forums I visit, that are specifically a bookmark to the forum with query string settings so that I see only threads replied-to over the last day, but sorted by number of replies, ascending. In other words, while I don't see older threads, I see the threads
that have been replied-to the least at the top of my forums. I do this deliberately because those threads are typically threads (1) that are questions for which no one provided an answer, and perhaps I could, (2) that are an interesting discussion just getting started, and (3) that are least likely to, if controversial, have gotten past the point where the thread degrades into petty sniping. (I typically unsubscribe from threads after Page 4, so no sense in subscribing to a thread I'll be unsubscribing to in no time.) So actually, bumping a thread, which adds a new reply, makes those threads go further
down in the forum, from my standpoint.