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.