Rationale: 1. Fetching the latest pubsnumber can never result in a wrong outcome. 2. 99% of the time it will in fact actively save the author from the hell of manually syncing pubsnumbers. 3. The overhead of fetching the latest pubsnumber as a percentage of the whole process is a micro portion of 1% (downloading topics, pushing to brew etc...). 4. Typing --fetch-pubsnum every time is 80% more work.
Point one is incorrect actually, if the --fetch-pubsnum is specified then it will fail if the URL doesn't exist. Other then that the only argument I have against it is that not all builds might be published using brew. It was for those two reasons that I didn't make it the default in the first place (as I've mentioned in Bug #829947 already). Anyways I'll re-evaluate it and talk it over and get back to this RFE.
Since the pubsnumber is no longer valid in Publican 3, this functionality is no longer usable and therefore would be a waste of time. So I'm marking this as closed.