Which claim are you questioning here? That they are ad-hominem *or* that ad-hominems will make the person defensive *or*that making someone defensive makes them less likely to listen to reason?
As far as what I'm assuming, well... have you ever tried telling someone that they are being stupid or dishonest during an argument, or had someone do this to you? It pretty much always goes down as I described, at least in my experience.
There are certainly situations when it's appropriate, and I do it with close friends and appreciate it when they call out my stupidity and dishonesty, but that's only because there is already an established common ground of mutual trust, understanding and respect, and there's a lot of nuance in these situations that can't be compressed into a simple causal model...
If you make someone defensive, they are incentivized to defend their character, rather than their argument. This makes it less likely that you will hear convincing arguments from them, even if they have them.
Also, speech can affect people and have consequences, such as passing on information or changing someones mood (e.g. making them defensive). For that matter, thinking is a behavior I can choose to engage in that can have consequences, e.g. if I lie to myself it will influence later perceptions and behavior, if I do a mental calculation then I have gained information. If you don't want to call thinking or speech 'action' I guess that's fine, but the arguments for consequentialism apply to them just as well.
What other reasonable purposes of arguing do you see, other than the one in the footnote? I am confused by your comment.
I guess that's possible, but why is that my problem?
Why are you arguing with someone if you don't want to learn from their point of view or share your point of view? Making someone defensive is counter productive to both goals.
Is there a reasonable third goal? (Maybe to convince an audience? Although, including an audience is starting to add more to the scenario 'suppose you are arguing with someone.')
This seems like a potentially counter productive heuristic. If the conclusion is that the person who is 'wrong' is either 'stupid' or 'dishonest' you are establishing an antogonistic tone to the interaction.
There are several other arrows pointing to disagreement (perception of 'wrongness'), including different interpretations of the question or different information about the problem. Making it easier to feel certain that someone is either 'stupid or dishonest' doesn't seem like a helpful way to move on from disagreement, since these are both ad-hominem and will make the other person defensive and less likely to listen to reason...
There are of course situations when people are wrong, but I don't think it's useful to include only two arrows here. For instance, if a mathematical proof purports to prove something that is known to be wrong, then another arrow is just something like "made a subtle calculation error in one of the steps," which is not exactly the same as stupidity or dishonesty...
I see what you are saying. I think an assumption I'm making is that it is correct to say what you believe in an argument. I'm not always successful at this, but if my heuristics where telling me that the person I'm talking to is stupid or dishonest, it would definitely come through the subtext even if I didn't say it out loud. People are generally pretty perceptive and I'm not a good liar, and I wouldn't be surprised if they felt defensive without knowing why.
I'm also making the assumption that what the OP labels as wrongness is often only a perception of wrongness, or disagreement. This assumption obviously doesn't always apply. However, whether I perceive someone as 'wrong' or 'taking a different stance' has something to do with whether I've labeled them as stupid or dishonest. There's a feedback loop that I'd like to avoid, especially if I'm talking to someone reasonable.
If I believed that the person I was talking to was genuinely stupid or dishonest I would just stop talking to them. Usually there are other signals for this though, although it's true that one of the strongest signals is being extremely stubborn about easily verifiable facts.