The thing is to call people names (and mention irrelevant traits, etc.) as part of strategy.

Why do people do things? Who knows. Do you have particular examples other than the "bloody idiot"? The line between opinion and informal argument is not clear cut, in fact people instinctively indicate some basis for their opinion in conversations, maybe not well articulated but discernible in context, and to the extent that getting from it to the opinion relies on inferences, cognitive biases, etc., it can involve informal (not necessarily "logical", i.e. formal) fallacies.

insulting is not an argument so if someone is making an argument and you are not, then you have yielded.

"Opinons are like rear ends: everybody's got one". So if you shove yours — that is quite honestly not very pretty, poorly packaged, and on the whole not even intended to be seen as pleasing — right into someone's face when you were having a (supposedly) serious discussion... what kind of reaction did you expect?! Happiness? A concession to your stance? Consensus?

Is it a typo that Discussion A says "healthy" where Discussion B says "unhealthy"? (Otherwise I'm unsure of the significance of this.)

@Dario If somebody is making an argument so stupid it does not warrant engagement, calling them a bloody idiot is definitely not a case of the ad hominem fallacy. In my experience, people will label any insult in a discussion "ad hominem" because they think it's a fancy name for insult.

@ChrisG If an argument is "obviously faulty", then it is trivial to provide an argument as to why the argument is faulty. If you then attack the person, instead of stating this trivial counter-argument, then perhaps their argument was not as obviously faulty as you think. To make an analogue: if the goal-keeper decides to leave the goal in the middle of the match, providing you with an opportunity for easy points... should you then start insulting the goal-keeper for being silly, or just lob the ball into the goal and thank them for the points?

@MichaelK I'm talking about situations in which the person making the stupid argument is acting in bad faith. Making a counter-argument here means you have lost. Have you ever had an argument in a youtube comment section? (also, in my previous comment, "people" = "some people")

@ChrisG How are you arguing for that insulting someone that acts in bad faith in any way is a "win"? What gain is there for me to start characterising them in a derogatory manner? Also there is a huge difference between a (reasonably) sensible discourse — where people actually know what a logical fallacy is and what and "ad hominem" means — and the ever-ongoing train wreck that is the YouTube comment section.

@ChrisG If you are aware that the YouTube comments section is not a sensible discourse, but the question — obviously — pertained to sensible discourses, why did you even bring it up? My comment stands, with a slight modification: if an argument — in an otherwise sensible discourse — is obviously faulty, then there is more to be gained for me to give a trivial counter-argument than to insult them. I cannot win any discussion, sensible or not, with insults. I say that adding insults to a discourse drags it down to YouTube comments section level.

Let us continue this discussion in chat.

The fallacy that you assume that someone else wants to be insulted or that insulting would get you anywhere. This question could also belong to IPS.SE.

"...you will find that most people on the internet will describe both responses as ad-hominems." [citation needed] Seriously, if this is happening to you that frequently, you might need to re-evaluate your communication style or who you are communicating with.

The principle of charity requires us to assume that your insult is intended to be an argument, fallacious though it may be. If it's not intended as an argument, then you're just a bully.

I think the more interesting case is "(a) Smoking is healthy (b) No, we have thousands of studies saying otherwise, and you're an idiot." Sometimes the insult will accompany a perfectly valid argument. In a sense the insult is pointless and adds nothing, or is even distracting; in another sense, it would be wrong to disregard the argument because of the insult, and... maybe insults are warranted sometimes.

It might be useful to look at the implicit argument you're making in Discussion A: "You're a bloody idiot. In the past, arguing with idiots has usually wasted my time and provided no benefits. Therefore, if I argue with you, the odds that I will be glad I did are slim. So I won't." That's a valid argument. A probabilistic one, certainly, but valid.

@kbelder, well, how about, "You're an idiot for arguing from authority, and your doctor is probably a quack?" :P

'Insult=ad hominem' is easy to understand, so gets thrown around. In truth, ad hominem requires your claim to rest solely on the attack for its foundation. 'You are wrong because you are stupid' is ad hominem. 'You are stupid and wrong because of X, Y, and Z' is not ad hominem nor fallacious. It's just rude. And rude people can speak truth (as well as can the stupid which is why ad hominem is a fallacy).

It's only a fallacy if you're basing an argument off of that insult that intends to be logical and free from flaws. Some ad hominens might not be fallacious in arguments about someone's character and morals, but it's easy to get caught up in the wrong details that way. At the very least in the case above, ad hominens are poor form because they include extraneous information. For example, "Jill is a patsy, and patsies are weak, this is why patsies are weak, this is why Jill is a patsy, therefore Jill should not be expected to be strong" really doesn't benefit from calling Jill a patsy at all.

Another thing to watch out for with this sort of thinking is that fallacy fallacy. You can't discredit someone's conclusion because their argument is fallacious. For example, 1+2 = 1+1+1+1 = 3. Here the sum of 1 and 2 is not equal to the sum of 4 1's, but it is equal to 3. When someone accuses you of using a fallacious argument keep in mind that they're attacking your logic and not your premise or conclusion. It's bad form to dismiss someone because they use ad hominens or because they've brought up something that's not related to the discussion. That's just an excuse to leave a discussion.