Phil Plait’s got a serious take-down of the recent claim that there’s been a meteorite found that has diatom fossils in it (at Salon).  Plait’s case is along a few lines: (1) that the rock doesn’t look like it’s a meteorite and has no documentation of how it was found or recovered, (2) the diatoms in it seem to be from Earth, like from a riverbed.  But he opens by criticizing the source of the claim.  He says  N. C. Wickramasinghe, the author of the paper reporting the meteorite, “jumps on everything, with little or no evidence, and says it’s from outer space, so I think there’s a case to be made for a bias on his part.”

Plait then turns to forearm against a concern about the present line of argument:

Now, you might accuse me of using an ad hominem, an argument that cast aspersions on the person making the claim, and not attacking the claim itself. I’ll get to the claim in a moment, but sometimes an ad hominem is warranted!

He makes the case with an analogy:

If Jenny McCarthy claimed botox cures autism, again, you might be forgiven for doubting it based on her previous anti-vaccine and other false claims. You still need to examine the claims on their own merits, of course, but: Fool me once, shame on you; fool me twice, shame on me.

But, now, this isn’t an ad hominem, per se, is it?  When the premises are that the person has a bad track record in the area they are reporting in (or in relevantly similar areas), that’s not ad hominem, but a case against their status as an authority.  I suppose that the basic thought is:  arguments against the person are appropriate when they are relevant to whether the conclusion is acceptable.  If we have reason to believe that S is unreliable, that’s a relevant consideration when we’re considering S’s reportage.

So a question to the NS readers:  should we save terms like ad hominem exclusively for the fallaciously irrelevant considerations of a speaker to impugn his/her claims, or can we allow the term to extend to relevant considerations?  I’ve argued that we should have that flexibility with plenty of other forms of argument, even with straw men and the tu quoque.  But ad hominem seems to have exclusively fallacious connotations for me.  Thoughts?