Bumping an old thread is perfectly acceptable IF you have a good reason.
Bumping a 5 year old thread to ask where the islanders are in a MOC doesn't make sense. The chances that the MOC is still put together is slim. Therefore that doesn't make alot of sense to do.
Now if you bump an old topic and have a relavant question and the discussion can easily be started up again, then that's fine.
This thread for example is a perfect candidate for being bumped. Non-Pirate Mini-Figures Useful for Pirate MOCing? Simply becasue the discussion is timeless even in 4 years from now, someone could easily spark up discussion on MOC'ing only pirate figs or blending regular figs in as well.
Something like this PETITION to keep the Pirates theme beyond 2010 and beyond... would not make sense to bump in say 3 years becasue well, it will just be too old, we obviously now know the result of the year 2010, so what would be the point.
Please note, these are not the official rules of EB, just my personal advice on the issue.