(I hope that is the least click-baity title ever.)
Political topics elicit lower quality participation, holding the set of participants fixed. This is the thesis of "politics is the mind-killer".
Here's a separate effect: Political topics attract mind-killed participants. This can happen even when the initial participants are not mind-killed by the topic.
Since outreach is important, this could be a good thing. Raise the sanity water line! But the sea of people eager to enter political discussions is vast, and the epistemic problems can run deep. Of course not everyone needs to come perfectly prealigned with community norms, but any community will be limited in how robustly it can handle an influx of participants expecting a different set of norms. If you look at other forums, it seems to take very little overt contemporary political discussion before the whole place is swamped, and politics becomes endemic. As appealing as "LW, but with slightly more contemporary politics" sounds, it's probably not even an option. You have "LW, with politics in every thread", and "LW, with as little politics as we can manage".
That said, most of the problems are avoided by just not saying anything that patterns matches too easily to current political issues. From what I can tell, LW has always had tons of meta-political content, which doesn't seem to cause problems, as well as standard political points presented in unusual ways, and contrarian political opinions that are too marginal to raise concern. Frankly, if you have a "no politics" norm, people will still talk about politics, but to a limited degree. But if you don't even half-heartedly (or even hypocritically) discourage politics, then a open-entry site that accepts general topics will risk spiraling too far in a political direction.
As an aside, I'm not apolitical. Although some people advance a more sweeping dismissal of the importance or utility of political debate, this isn't required to justify restricting politics in certain contexts. The sort of the argument I've sketched (I don't want LW to be swamped by the worse sorts of people who can be attracted to political debate) is enough. There's no hypocrisy in not wanting politics on LW, but accepting political talk (and the warts it entails) elsewhere. Of the top of my head, Yvain is one LW affiliate who now largely writes about more politically charged topics on their own blog (SlateStarCodex), and there are some other progressive blogs in that direction. There are libertarians and right-leaning (reactionary? NRx-lbgt?) connections. I would love a grand unification as much as anyone, (of course, provided we all realize that I've been right all along), but please let's not tell the generals to bring their armies here for the negotiations.
Well, I think it's true, interesting, and useful :)
The argument is a specific case of a more general form (explaining changing group dynamics by selection into the group, driven by the norms of the group, but without the norms necessarily causing a direct change to any individual's behavior) which I think is a powerful pattern to understand. But like a lot of social dynamics, explicitly pointing it out can be tricky, because it can make the speaker seem snooty or Machiavellian or tactless, and because it can insult large classes of people, possibly including current group members. I felt that LW is one of the few places where I could voice this type of argument and get a charitable reception (after all, I'm indirectly insulting everyone who likes to talk politics, which is most people, including me :P)
To be clear: I don't think lesswrong is currently being hurt by this dynamic. But I do see periodic comments criticizing the use of only internal risks (mind-killing ourselves) as the justification for avoiding political topics. I'm sympathetic to some of these critiques, and I wanted to promote a reason to avoiding political topics that didn't imply that mind-killing susceptibility was somehow an insurmountable problem for individuals.
Homeostasis of social communities is a very interesting topic. Let me just point out that there are dangers on all sides -- you don't want to be at the mercy of every wandering band of barbarians, but you also don't want to become an inbred group locked up high in an ivory tower.