As a civilization and in our society, we systematically doubt the testimony of interested persons. From ancient times, rhetoricians noted that interest undermines credibility. From then until the present, the formal practice of doubting interested testimony has played a role in the law of evidence. It is still embodied today in many areas from jury instructions to standards of summary judgment. I neither think, as you say, that I am starting this line of reasoning nor do I fear that it leads nowhere good.
We doubt self-interested testimony because, whether there is an intentional lie or not, interest shades both the recollection and framing of events. We are weak in judging and remembering things that strongly affect us. Of course, in formal logic, the interest of the speaker cannot be raised against an argument. But practical judgment proceeds by different rules in dealing with testimony.
There is, I think, a fairly straightforward confusion here. When we talk of an 'interested person' or a 'personal interest' we may mean two entirely different things:
(1) this person has a prejudice potentially able to be a source of falsehood in the testimony they give (let's call this a distorting interest)
(2) this person has a personal stake in the matter with regard to which they are testifying (let's call this a personal stake)
The two are not the same thing at all. (1) is a matter of subjective distortions; (2) is a matter of objective relevance. It is clear in the context -- Lindemann's testimony about a professor's remark about woman -- that Pensans has no evidence of distorting interest. The only reason given for suggesting that Lindemann has a distorting interest is that
It is strongly in her interest that there be lots of evil resistance to women philosophers. Otherwise, the Women-Philosophers project sounds like a pathetic special-interest group.
As I pointed out, the assumptions behind the 'otherwise' are problematic at best. But let that be set aside. Is this a serious argument for distorting interest? No. The only point it brings forward merely shows that Lindemann has a personal stake in the Women Philosophers project.
As I've noted before, with Hume's use of Pensans's principle in his arguments against miracles, the sort of principle presented here, combined with the conflation, leads to utter absurdity. Experts by definition have a fairly hefty personal stake in most of the public claims they make, because they put their reputations and sometimes their careers on the line with them; if this personal stake were treated as a distorting interest, it would require us always to doubt the testimony of experts when pitted against those with less personal stake -- quacks and conspiracy theorists, for instance, or Joe at the bar who is just mouthing off.
Another example: if someone is accused of a crime, but gives his own account of events in which he was framed, it would be utter stupidity for a newspaper columnist, for instance, to dismiss it by saying, "Of course, it is strongly in his interest to claim that someone framed him. Otherwise, he'll go to jail. So there is some ground for doubting his testimony." But the journalist has provided no ground for doubting his testimony at all, only a reason for thinking it's very important matter which way the ultimate decision goes. The only grounds for doubting his testimony are reasons for thinking the testimony might be false. But the purely hypothetical scenario that if he's guilty he would have reason to lie gives us no reason whatsoever to think one way or another about the man's testimony. It is a purely hypothetical scenario, and it can only provide reasons for anything if we have reasons suggesting that it might not be so hypothetical after all.
Consider another case: Some members of one of the poorer black communities in a large city come forward with testimony of harassment and racist actions by the local police. And someone, hearing about this, but knowing nothing about the matter except the testimony itself, says, "Of course, there is some ground for doubting their testimony. They have a strong interest in claiming that the police are racist; otherwise they have to take the blame for the fact that so many of their members are getting into trouble with the police." What has happened here? Is it not, in fact, that the person who says this has committed an injustice against the people who gave the original testimony, by casting aspersions on their claims on the basis of no evidence at all? Testimony should not be treated lightly. Pensans is right that practical judgment enters into the matter; but whenever practical judgment enters into the matter, so does ethics. And testimonial injustice is injustice.
When we recognize this, we see that Pensans is wrong to say that the principle is applied systematically. If no distinction is made between distorting interest and personal stake, it is not applied systematically at all, and could not possibly be short of society going insane; rather, it's applied when people find it convenient to short circuit serious investigation or to cast aspersions on other people. Instead of being evidence-based, it involves people making things up on the basis of purely hypothetical situations that they can't prove. If we have evidence that there is a distorting interest, then we can cite it as reason to doubt the testimony. Under certain circumstances (police investigations, historical research, etc.) we may investigate the question of distorting interest as a matter of course for each bit of testimony we come across, because we know by experience how much it can lead that sort of investigation astray. Of course, in the Lindemann case, none of those circumstances obtain. Lindemann brings up the personal anecdote for no other reason than to explain why she's interested in the history of women in philosophy. And thus Pensans's whole argument in this particular case turns out to be an argument that we have grounds to doubt her testimony about how she became strongly interested in the subject because she's strongly interested in the subject, which is absurd.
But even outside this case, suggesting distorting interest without evidence is always unjust, and what is more, it is dangerously close to being a pretense that one can, God-like, peer into the hearts of others, even if only probabilistically, without having to bother with the external signs. And that would be hubris. That's something we need to take steps to avoid in all our argument and reasoning.
Incidentally, it's important to recognize (although it often is not) that the personal stake in an argument raises ethical questions about how we handle ourselves in reasoning and discussion for all of reasoning, not just inference from testimony. To the extent that a person has a personal stake in the point we are arguing about, we must take that into account in the way we publically reason and argue about it, because their personal stake raises the risk, which must be avoided or at least compensated for, of treating them unjustly in argument. In many particular cases, these ethical issues are easy to miss, because we can presume or postulate that everyone has a strong personal stake in the truth whether they like it or not; but the ethical issues are not nonexistent, and the truth should not be put forward as an excuse for mistreating people during an argument.
I raise the point because it's clear that we all, and especially academics (who learn to argue in a conclave where it's in everyone's interests to allow truth an even greater trumping force than it ordinarily has), have difficulty engaging in the ethical self-critique that responsible (and therefore rational) argument requires. And so it seems to me worth mentioning it so that we might all, and this includes myself, have a reminder that this self-examination continually needs to be there if we are to be rational in our arguments at all.