By what kind of criteria do you estimate whether something is a "gain" or not and whose gain is it?
What kind of criteria? Depends on the something. (Again a general answer but again the question is general. The criteria for evaluating whether to take an aspirin are different to those for evaluating an online debate, which are different again to those for evaluating some bookshelves, which are different again to...)
Whose gain? Whoever the person doing the evaluating cares about.
And if the answer is "look at utility", the question just chains to how do you estimate whether something is positive-utility,
Well, you hit rock bottom eventually; you translate things into consequences about which you have reasonably clean-cut "this is good" or "this is bad" intuitions. Or, if you're doing it in a more explicit cost-benefit-analysis kind of way, you can pin rough conversion factors on each of the final consequences which re-express those consequences in terms of a single numeraire for comparison.
especially with non-obvious issues like having or banning certain kinds of debates on a forum
Here I think the estimating is relatively easy, because I'm weighing up "We should debate what needs to be debated", apparently in the context of LW specifically, and the impression I got from your phrasing was that you were implicitly excluding broad classes of consequences like warm fuzzy hedons. If so, considering the issue on your terms (as I understand them), I can simplify the calculation by leaving out hedonic and similarly self-centered aspects.
Elaborating on why I interpreted you like that: when people use the language of duty or obligation, as in "We should [X] what needs to be [X]ed", they normally imply something like "we need to do that, even if through gritted teeth, for prudential reasons", rather than e.g. "that would be fun, we should do that". If that's what you meant here (perhaps I misunderstood you?), that excludes consequences like the little glow we might get from signalling how clever we are by debating things, the potential pleasure of learning things that're new to us, or even the epistemic virtue of inching closer to the right answer to a knotty, politically polarized empirical question.
Once one rules out those kinds of consequences, the main kind that's left, I reckon, is how those debates lead to resolutions, or at least lessening, of political problems in the wider world. (If our debates didn't lead to such improvements, then what would be obliging us to "debate what needs to be debated"?*) And I'm sceptical political debates on LW would accomplish that, at least on average.
I'm pretty sure some people would disagree with me. I'm also pretty sure it's at least debatable (haw) whether political debates on LW would improve actually existing politics, and whether effort spent on those debates would be better spent on something else (like arguing politics with people known to have influence) and that's enough for my point to go through. In fact, I'm now a little more surprised by your original comment, since your questions suggest you have difficulty working out whether "having or banning certain kinds of debates on a forum" is on balance a good thing or not, which I'm not sure how to square with your confident judgement that "We should debate what needs to be debated".
* One way I could be misunderstanding you: perhaps you do take utility-maximizing consequentialism seriously enough that you actually think the e.g. entertainment value of arguing outweighs the other consequences of arguing here, and so we're morally obliged to debate politics here for the entertainment value. I don't have the impression you're of that view, though.
Thanks for the serious answer.
perhaps you do take utility-maximizing consequentialism seriously enough
No, I do not.
entertainment value of arguing outweighs the other consequences of arguing here
...but that is a very interesting idea :-D
(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.