Perhaps willingness to play dirty signals commitment, and I expect some of the time people are more interested in showing off their commitment than actually making progress toward the putative goal.
Yeah, that's pretty much my take. Often, signalling the willingness to play dirty without actually doing so gets us the collective benefits of "niceness, community, and civilization" while also getting us some extra individual benefits on top of that. And asserting that playing dirty is effective and that rational agents should be willing to play dirty can be an effecting way of signalling that willingness.
Until someone comes along reads all the stuff you wrote about the importance of playing dirty and believes you.
Scott, known on LessWrong as Yvain, recently wrote a post complaining about an inaccurate rape statistic.
Arthur Chu, who is notable for winning money on Jeopardy recently, argued against Scott's stance that we should be honest in arguments in a comment thread on Jeff Kaufman's Facebook profile, which can be read here.
Scott just responded here, with a number of points relevant to the topic of rationalist communities.
I am interested in what LW thinks of this.
Obviously, at some point being polite in our arguments is silly. I'd be interested in people's opinions of how dire the real world consequences have to be before it's worthwhile debating dishonestly.