I stand behind the entire conversation about ad hominem. It accomplishes nothing to accuse one of using it. It fails to refute anything. It is often ad hominem itself. All it does is call attention to a logical fallacy that is unrelated to the statement being made.
Like this:
Joe's idea stinks to high heaven. Afterall, his father's an idiot.
Bad logic, to be sure. But it makes no difference with regard to the truth of untruth of Joe's idea. The statement about Joe's father is irrelevant. Ad hominem points that out, but nobody cares.
It's bad logic, and that's *all* it is. All you can do is point out that it's bad logic. If someone makes an argument that you admit is bad logic, why wouldn't the rational thing to do be to simply point out that it's bad logic? You're suggesting essentially that you should ignore the validity of an argument someone else made and instead change the subject. That's crazy.
Consider this:
Joe's idea stinks to high heaven. Oh, and by the way, I want a burger for lunch.
So he wants a burger for lunch. Good for him. Are you going to try and point out the untruth of Joe's idea stinking to high heaven by arguing about the speaker's hunger pains? If so, you're only engaging in deflection and nonsense. Instead, you should stick to the topic, which is whether Joe's idea stinks to high heaven or not.
Sure, in this example, that's right. But if they said "Joe's idea stinks to high heaven because I want a burger for lunch", all you can do is point out that hunger pains have nothing to do with Joe's idea. That's id.
You will accomplish nothing by discussing lunch, Joe's father, or whether the speaker mentioned lunch or Joe's father.
Sure, in your ridiculous example.
If on the other hand, you verbally insult me personally by engaging in ad hominem, then it might be worth my time to discuss it with you, but not because it has anything to do with the main point of argument, but because, by insulting me, I may wish to insult you back, argue the point of the insult, or plant a fist in your face.
Pointing out that an argument is ad hominem is attacking the validity of the argument. The crux of the ad hominem fallacy is that you address the character of the person who made it, not the argument. For example: "Because Joe makes ad hominem arguments, we can reject this argument" is ad hominem. It addresses Joe's character rather than the validity of his argument. But "Because Joe's argument is ad hominem, we can reject it" is perfectly valid. It doesn't address Joe's character but the validity of his argument.
We can, and should, refute invalid arguments. But we can't dismiss an argument based on characteristics of the person who made it. But we must evaluate an argument based on characteristics of that argument itself.
Your suggestion is basically that we presume bad faith on the part of the person advancing the argument because it's invalid and not point out that it's invalid. I'm suggesting we presume good faith on the part of person advancing the argument and do them the courtesy of pointing out that their argument is invalid and that they should honor their good faith, accept its invalidity, and abandon it. Not addressing the specific argument they actually made by pointing out that it's fallacious will make it seem to them that you're simply ignoring the arguments they actually made and just making unrelated arguments. That will make them find it hard to presume you are arguing in good faith and it's a recipe for turning what could have been a productive discussion into chaos.