In my experience very few people will listen to an argument after the person presenting the argument has called them stupid. When you call somebody a moron, then i expect that you've drastically reduced the chances that this person will listen to you.

In other words, the action of calling someone a moron takes convincing the target off of the table, if you haven't done that already.

My guess is that, when you call you're in a debate and you call your opponent stupid, it's mainly for the benefit of the people who already agree with you; the main purpose is probably designating "which side you're on" rather than convincing anyone who disagrees. This reminds me of the line of retreat idea -- it's easier for people to change their minds if they can do so without calling themselves stupid.

Agree, second, and applaud.

I've mostly thought of this as a matter of economy. To quote something I wrote quite a while ago, elsewhere:

In general, the things we don't like, don't want, won't tolerate vastly outnumber the things we do like, do want, do find acceptable. To enumerate them all would be a huge waste of time. And to name, for instance, only one thing we don't like - that is just getting started on the enumeration.

So, we communicate more efficiently by saying what we like, what we want, what we prefer, how we'd change things for the better, and so on.

Being rude is essentially being negative, with an additional effort to deliberately violate the conditions of felicity of whatever form of conversation is being used.

You could also cite (though I can't dig up a link right now) those studies recently published claiming that feeling rejected lowers people's IQ by a substantial amount.

Not an appropriate thing to do when you're trying to leverage collective intelligence; and if your aim isn't to leverage collective intelligence, why are you wasting your valuable time and talents posting on a public forum ?

I'm going to repeat my request (for the last time) that the most recent Open Thread have a link in the bar up top, between 'Top' and 'Comments', so that people can reach it a tad easier. (Possible downside: people could amble onto the site and more easily post time-wasting nonsense.)