While bumps are generally good, there are niche cases where they are harmful:

I recently decided I would consolidate two tags on Stack Overflow: "openoffice" and "openoffice.org". They refer to precisely the same thing, and having two tags for them just fragments the organizational structure.

However, after a couple of retags, I noticed that these questions were being bumped to the front page, clogging it up. Not wanting to be disruptive, I stopped retagging.

This prevents a very useful organizational benefit of SO's community structure. As well as consolidating two identical tags, I could do things like look at each question in "openoffice.org", and add the tag "openoffice-writer", "openoffice-calc", etc. where appropriate. Abuse would be unlikely, due to the 500 reputation minimum for retagging questions.