-- i.e., so long as you're not resting your argument on the ad hominem information, but including it because it seems relevant for casting doubt upon the author's credibility or lack of bias.
I think it is intellectual cowardice to resort to ad hominem attacks, especially if one is communicating this information to other intellectuals and/or academics. I say this because in open intellectual circles such as RG one is dealing with ideas, theories, etc.; and it is these that should be the focus when mounting challenges in order to disagree (or agree) with the written word. It is my experience that ad hominem attacks sidetrack one from confronting the issues raised by an author's intellectual contribution and thus are a hindrance to pure objective thought and reasoning. In short, it is not worthy of you to waste your time attacking the person, rather concentrate on her ideas, theories, hypotheticals, visions, arguments, etc., and this will expand your intellectual prowess. Leave ad hominem attacks to the members of Congress -- Lord knows, they are the experts in this field.
The use of ad hominem arguments may be appropriate depending on whether context makes it relevant, but is rarely appropriate in academic publications. This is because the ad hominem argument is usually classified as a fallacy of relevance because facts about a person making a claim are rarely relevant to arguments for or against that claim. However, much like the argument from authority, there are cases where facts about the person making the claim can be relevant. For the argument from authority, if a person making a simple factual claim has relevant expertise to back up that claim, then that expertise provides some reason to believe the claim (minimally, we tend to believe scientists' reports of their own raw data - at least enough to make it worthwhile to try to replicate an experiment). Likewise, for an ad hominem argument, pointing out that a person has no expertise in the subject area in which he or she makes a simple factual claim tends to remove reasons to believe the claim. So, an ad hominem argument can in some cases appropriately address someone's credibility in making a specific claim. Of course, neither the argument from authority nor the argument ad hominem should be taken as decisive.
Also, something like ad hominem arguments may be used as explanations. In cases where the position being criticized by legitimate arguments is a particularly weird and/or surprising position, it is sometimes relevant to explain why a person happens to hold that position. Indeed, it may sometimes be necessary to explain why a person holds a surprising position in order to convince one's audience that someone actually believes something quite peculiar. In such cases, one should be careful to make it clear that the explanation offered has no bearing on the validity of the surprising claims.
I am not sure whether one situation qualifies as being "to the man" (ad hominem) or not. What do you do when someone claims an expertise in a field which they clearly do not have? Is it an ad hominem form of argumentation to point out that this author appears to be speaking out of ignorance, or must one engage each of their arguments? We live in a day when people with an assortment of letters behind their names (denoting their degree) seem to think that their degree in one field qualifies them to hold forth about all fields, and who frequently demonstrate an embarrassing and almost painful ignorance of the subject which they are engaging. Is it ad hominem to say that the person is clearly unqualified to address the issue at hand?
I have done the following a couple of times: I first judge the work as such. And only once I have come to a critical judgement (and nothing moves me from there), then I dare look at some information concerning the person.
By that I mean either to nuance my concept, or else to re-affirm it. But it's just a matter of nuance - and my judgment does not change by that "ad hominem" information.
The ad hominem information provides personal, or social, or cultural lights/shadows. - (After all, I mean just between brackets, science is done by human beings. Yet, science cannot be reduced to the humanity of people).
Based on Stanford Encyclopedia of Philosophy, we have three major types of ad hominem.
"The ad hominem fallacy involves bringing negative aspects of an arguer, or their situation, to bear on the view they are advancing. There are three commonly recognized versions of the fallacy. The abusive ad hominem fallacy involves saying that someone’s view should not be accepted because they have some unfavorable property.
Thompson’s proposal for the wetlands may safely be rejected because last year she was arrested for hunting without a license.
The hunter Thompson, although she broke the law, may nevertheless have a very good plan for the wetlands.
Another, more subtle version of the fallacy is the circumstantial ad hominem in which, given the circumstances in which the arguer finds him or herself, it is alleged that their position is supported by self-interest rather than by good evidence. Hence, the scientific studies produced by industrialists to show that the levels of pollution at their factories are within the law may be undeservedly rejected because they are thought to be self-serving. Yet it is possible that the studies are sound: just because what someone says is in their self-interest, does not mean it should be rejected.
The third version of the ad hominem fallacy is the tu quoque. It involves not accepting a view or a recommendation because the espouser him- or herself does not follow it. Thus, if our neighbor advises us to exercise regularly and we reject her advice on the basis that she does not exercise regularly, we will commit the tu quoque fallacy: the value of advice is not wholly dependent on the integrity of the advisor."
So here is my take on it: The first one (i.e the abusive ad hominem) is wrong, because it is totally irrelevant to the conversation and deviates the attention from the main substance.
As for the second and the third types, I believe they are not sufficient to reject an argument. However, you ask a different question: "Is it ever OK to include "ad hominem" information about an author whose work you are critiquing?"
I think it might contribute to the conversation if you add the info on the circumstantial ad hominem (which is basically conflict of interest information). This is what is basically required by the journals to improve the "quality" of research and to reduce the effect of commercial and financial motives on research
As for the third type, it might add to the conversation by questioning the seriousness of the author's work (I am actually uncertain about this one).