[baddox]

> But when somebody jumps into a discussion with a few sentences, asking who they are and what their motivations are is much more reasonable.

Not really. You're not taking the idea of ad hominem far enough. When somebody jumps into a discussion with a few sentence and no citations or evidence, you ought to reject the validity of that argument regardless of who is presenting the argument. That argument is unsubstantiated, period, whether it's posted by a brand new throwaway account or a renowned expert in the area of discussion.