If I call Carrol an asshole, everyone knows i'm being rude.
If I chose to make his argument into a strawman or bait him or speculate about bad motives or maintain that he's a screeching homeless guy typing in from the library and he's not a retired professor at all, then what have i done?
1. refused to engage the substance of his argument. Right off the bat, that tells Carrol I have no respect for him. I do not think he is worthy of a considered response.
2. When it's ad hominem, it puts Carrol in the position of having to defend his person against a lie or let the lie stand. He's screwed either way. It's an unprincipled debate tactic that should be used against enemies and not among people on your side.
When the law professor and blogger, Ann B, decided to spread the rumor that I was really an academic and that I was a paid informant for the Right Wing, this got picked up by people who trusted her. They blogged about it.
Had she merely called me an asshole, everyone would have known that she was being mean. But when she engaged in logical fallacy, people who trusted her believed her. I was put in the position of defending myself from the lie, a long winded response indeed, or letting the lie stand. I was screwed either way.
I elected to allow the lie to stand and I told the people who subscribed to my blog not to go swarm her blog with nasty posts. There are people, to this day, who apparently still think that I was a paid informant for the right wing.
That's why the two are different and it's why the use of LF, especially ad hominemn, is considered an unprincipled debate tactic among allies. We all use them, mostly accidentally. But that's why repeated use of them in argument sends the message to your interlocutor that you are more interested in scoring points than having a debate.
-- http://cleandraws.com Wear Clean Draws ('coz there's 5 million ways to kill a CEO)