I think, under some circumstances, it is acceptable.
For instance: If you ask a question that you really need answered fairly quickly and no-one responds then it is acceptable to bump it up a bit to try to bring it to the attention of someone who may not have been online earlier.
I've done it a couple of times when I was in a hurry and noticed someone who may be able to help is now online.
Also threads which contain important or interesting information can be bumped, like meet threads for example.
I don't know about others but I rarely look any further than the third page of a forum. In GD that could be as little as 12 hours. if you haven't been on the forums for a few days you could have missed something important and not know.
Having said all that, most threads which are interesting or contain important information are usually quite popular and as such, don't need bumping. One of the exceptions would be meet threads, as mentioned before, which tend to be started several months before the actual event and can go some time between spam fests and run the risk of pruning.
If one bumps for no other reason than to say "look at my thread people" then that is sad.
Bumping is, of course, against the rules in MM - and quite rightly so.
Stan