And even if we are good enough, and have a big rational discussion on a hot-button and usually divisive topic, and come out mostly agreeing that the Herp position is right an the Derp position is wrong - that will just make the site more attractive to less-rational Herpists, and give Derpists a pretext to dismiss LessWrong because "they're obviously motivated by Herpism".

This argument seems like it might show too much. If someone said this about an issue that isn't political (say the existence of God) we would reject it. What gives politics such a unique status? Certainly religious opinions create about as much tribalism.