I think we kind of got away with Bugzilla being obscure. Anyone theoretically could create an account, but they'd have to know where to find the Bugzilla, which wasn't linked from anywhere prominent, and they'd have to go through the process of account creation. GitHub is really really well known, and it's the first place people would think of looking and lots of people have accounts there already. I don't know if this is ever actually a problem for GitHub based projects; I hope you're right and we won't have a big problem with spam, trollish or plain clueless bug reports.