Maybe so, but this also assumes that you're good at determining who's an idiot. Many people are not, but think they are. So you need to consider that if you make a policy of "don't argue with idiots" widespread, it will be adopted by people with imperfect idiot-detectors. (And I'm pretty sure that many common LW positions would be considered idiocy in the larger world.)

Consider also that "don't argue with idiots" has much of the same superficial appeal as "allow the government to censor idiots". The ACLU defends Nazis for a reason, even though they're pretty obviously idiots: any measures taken against idiots will be taken against everyone else, too.