I agree that it's important to stay on top of findbugs problems, but it's also important that we properly indicate things that are problems before the patch and problems with the patch. otherwise folks will think of our warnings as false positives and stop listening to us.
I'm also torn on failing pre-patch over findbugs. Nightlies are really the place to flag issues with the main code base, though I get the advantage of how much more visible precommit QA is. Also, projects that want to be strict about findbugs status could always tie things into their main build so that pre-patch javac would fail with findbugs warnings anyways (and they could even do this just in QA by using the project patch process profile).
Anyhwo, I checked this version against HBase with
HBASE-13716 with and without the cli option and it behaved correctly in both cases, either just saying "everything is fine" or "before this patch there are 60-ish findbugs things wrong"