Worse than that, offlineimap can be plain destructive in the right circumstances. I've had it arbitrarily delete thousands of messages due to the local cache being in a strange state.
mbsync does not watch or poll for changes, but you can run mbsync in a loop from your shell. Or, if you are running Linux on the mail server and client and use Maildirs, you can run mbsync from mswatch (http://mswatch.sourceforge.net/). (Disclaimer: I'm the author of mswatch.)