*puts on Mod hat*
the way i understand it is, there's nothing wrong with bumping old threads if you have something to add to the topic.
the bumping of old threads is only frowned upon if people do it for the sole reason of bringing threads to the top of the page. i.e. the infamous 'bump!' post.
it is also frowned upon if people post in old thread with such things as 'i agree!' or something spammy, i.e. not contributing.
i may be wrong, i'm sure if i am one of the other mods will correct me if i am.