I prefer it to be forum stats or an event, milestone, etc but sometimes I can't be bothered telling them not to just post bump. So long as it's not being overdone.
I eventually plan to have that nice plugin that allows members to click to bump every so often be used on my forum, but for now I simply let them type bump if they wish. However, I think everyone would say that they would rather have people post some sort of content for a bump rather than just the word "Bump".