Your mistake is that you treat the situation as if I'm attacking some arguments by JEB_4_PREZ_2016. I am not. We are not discussing an object-level issue, we're discussing the consequences of choosing an unusual username.
Your mistake is assuming I am making that mistake, and arguing as if that were the case. You've started following a script; you think I've accused you of an ad-hominem, and are looking for the ad-hominem you made. A person comes before the Roman Senate wearing nothing but sandals, and presents a marvelous case on a currently discussed issue. The ad-hominem there took place before one of the senators, who disagreed with him, complained that he wasn't properly attired; it took place in the mind of the senators who were already discounting what he had to say, without having said a word against him.
In forming priors about his posts.
If that wordplay was deliberate, kudos.
And yep. So do you think ad-hominem is useful, then? (Of course it is. I'd rather read Scott Alexander's thoughts on something than Aaron Clarey's.)
Is it important that we notice it in a relatively controlled environment, so that, if it is in fact in error, we can correct for it when the heuristic leads us astray?
I'd say the answer is equally obvious.
it took place in the mind of the senators
Ah, so a precrime thoughtcrime ad hominem? :-D
I have a feeling you're extending the definition of ad hominem into places it wasn't meant to go. It is, basically, an invalid method of refuting an argument. It says nothing about which arguments to pay attention to, for example. Calling even a "useful" ad hominem something like considering the reputation of a speaker before deciding whether to allocate your valuable attention to his speech is a stretch, I think.
This thread is for asking any questions that might seem obvious, tangential, silly or what-have-you. Don't be shy, everyone has holes in their knowledge, though the fewer and the smaller we can make them, the better.
Please be respectful of other people's admitting ignorance and don't mock them for it, as they're doing a noble thing.
To any future monthly posters of SQ threads, please remember to add the "stupid_questions" tag.