One of the most annoying aspects of internet discourse is when someone enters a discussion and attempts to engage substantively with an ongoing debate, but is derided or ignored because s/he hasn't posted prolifically in the forum before. It doesn't matter if you've been lurking for years; it doesn't matter if you're right: you're not part of the group, and therefore not worth listening to.
If you are genuinely interested in examining an issue, not just in shooting the breeze with your friends, it shouldn't matter what the motives of the new entrant are. It shouldn't matter if you think that s/he might have just surfed over for the purpose of participating in a particular thread, or is actually in substantive disagreement with you on a variety of other issues, or is (heaven for-freaking-bid) not posting under his/her own name. If s/he makes a logical point that you would take seriously were it coming out of a regular's mouth, it behooves you to can the territorial, exclusionary insults and address the substance of the person's position.
If you're not willing to do that, just make your clubhouse atmosphere explicit so the rest of the world will know not to bother attempting to engage with you.
UPDATE: So this doesn't turn into a Carly-Simonesque repeat of my cryptic post on internet romances from a few years ago: the impetus for this post was something from a literary blog, but if you're asking "was it [some other type of blog]?" because of what you've seen there, there's probably a good reason for that. It happens in a lot of places, and I am sick of it. It's especially galling coming from people who claim to be interested in ideas.