In the discussion over whether there is something scandalous about AEI's effort to commission analyses of the IPCC report and proposed climate report, more than one commenter sought to defend the use of ad hominem arguments. For instance, "Justin" commented:

The "ad hominen criticism is wrong" argument is thorougly erroneous, as I have discussed before. When the reader, either because the evidence is withheld or too complex for him to understand, on a particular challenged argument, is expected to agree by trusting or distrusting the source, an ad hominen attack/ab hominen defense is all you have to go by.

In a bit of snark, I paraphrased this claim as "Ad hominem arguments are convenient for the lazy and uninformed." Even put this way, the claim has some truth, but all it means is that ad hominem evaluations have some utility as a time-saving, filtering device for those who lack the time or ability to evaluate the substance of a claim on its merits. It does not say anything at all about the truth or falsity of the claim itself. In other words, it does not establish that ad hominem arguments are valid arguments.

Put another way: While it may be reasonable in some contexts to say: "I trust information and arguments from X more than from Y because Y is [corrupt/bought/of the wrong ideology/etc.]", it is a logical fallacy to say "Y's argument is wrong" for the same reasons. The strength or weakness of Y's argument can be evaluated independently of Y's personal failings.

So, if I want to know something about the health risks of smoking, and lack the time, inclination, or ability to research the question for myself in any detail, I may decide to trust the word of a medical professional over that of a tobacco lobbyist — and I will make this decision because one is a medical professional who is concerned about encouraging good health and preventing sickness, while the other may have a financial incentive to gloss over the harms caused by his product. The truth or falsity of each person's claims, however, are independent of my evaluation. In this context, ad hominem information serves as a time-saving heuristic device, but that is all.

In the context of blog comment threads, I think it reasonable to presume that those who rely upon ad hominem arguments typically do so because they lack the time, inclination or ability to mount more substantive critiques. This is particularly the case where the argument is utilized for purposes beyond the dismissal of an appeal to authority. [After all, if one person in a debate wants to take the short-cut of appealing to an authority instead of spelling out an argument, it is reasonable to point out why the authority in question might not be so authoritative.] Blogs like this one aspire to be forums for reasoned discussion of various questions. Whether or not we succeed in our aims, that discussion requires engaging subjects on their merits, not resort to logical fallacies like the standard ad hominem.