A bump is never really acceptable except for emergencies. In most cases, people either don't respond because they don't know the answer or the question has already been answered a hundred times and they're tired of typing the answer. In the latter case, it is best to use the Search function and you'll likely find the answer.
As for reposting the same question over and over..that is considerred spam as well and will typically lead to the duplicate question topics being closed. It just causes too much clutter and disorganization. I hope this helps clarify why the rule is in place.
Jody