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.