Whether or not the lawful-goods of the world like Yvain are right, they are common. There are tons of people who want to side with good causes, but who are repulsed by the dark side even when used in favor of those causes. Maybe they aren't playing to win, but you don't play to win by saying you hate them for for following their lawful code.

For many people, the lawful code of "I'm siding with the truth" comes before the good code of "I'm going to press whatever issue." When these people see a movement playing dirty, advocating arguments as soldiers, where you decide whether to argue against it based on whether it's for your side rather than whether it's a good argument, getting mad at people for pointing out bad arguments from their side, they begin to suspect that your side is not the "Side of Truth". So you lose potential recruits. And the real Sith lords, not the ones who are trying to use the dark side for good, will have much less trouble hijacking your movement with the lawful-goods and their annoying code and the social standards they impose gone.

Leaving aside the honor among foes idea, and the "what if you're really the villain" idea, if your cause is really just, then although the lawful-goods are less effective than you, their existence is good for you. Not everything they do is good, but on balance they are a positive influence. You're not going to convince them to attempt to be dark side users for good like you are attempting to be, so stop giving them reasons to dislike you.

Even if you can convince them, the lawful-evils who think they are lawful-goods are listening to your arguments. Most people think they are good. It is hard to tell when you're not good. So the idea that only truly good people are bound by the lawful code is crazy. Lots of lawful evil is an unintentional corruption of lawful good, and this corruption doesn't unilaterally affect your goodness and your lawfulness. They could tell (or at least convince themselves) they weren't really good, if they didn't follow the lawful code, because they think like lawful good people in that respect. The lawful evil people who see you, and know you are opposed to them on the good/evil axis, think they see evil people saying "Forget this honor among enemies thing. We have no honor. Watch me put on this 'I am defectbot' shirt". And that is a much stronger argument to abandon the lawful code of rational argument and become the much more dangerous chaotic evil than what the lawful-goods hear, which is their chaotic good allies telling them to defect.

But in real modern human politics, it's more complicated because although there is one lawful/chaotic axis, there are many good/evil axes. Because there are many separate issues that people can get right or wrong. Arthur Chu thinks that the issue of overriding importance is social justice. So he demands that we drop all cooperation with people who are evil on that axis. He says we aren't playing to win. I can think of 3 issues (2 of them are actually broad categories of issues) that I am confident are more important than social justice, and which are easier to improve than the problems social justice wants to counter. In order of decreasing importance, existential risk, near-term animal suffering including factory farming and wild animals, and mortality/aging.

In real life, you don't demand that your allies be on the same end of every good/evil axis as you. That is not playing to win. A better strategy (and the one Chu is employing) is to pick the most important axis, and try and form a coalition based on that axis. Chu accuses LW of not playing to win, well, I'm just not playing to win along the social justice axis at the cost of everything else. I think different axes are more important.

And there's also the fact that for some causes, "lawful" people (people who play by the rules of rational discourse) are much better to have as allies. If we use bad statistics and dark arts to convince the masses to fund FAI research, they may as well fund Novamente as MIRI. Not all causes can benefit from irrational masses. Something like MIRI can't afford to even take one step down the path to the dark side. When you want to convince academics and experts of your cause, they will smell the dark arts on you and conclude you are a cult. And with the people you will attract by using dark arts, your organization will soon become one. The kind of people who you absolutely need to do object-level work for you are the kind of people who will never join you if you use the dark arts.

If you take a pluralistic "which axes are important" approach instead of the one that Chu takes, then there is a lot to be said for lawfulness, because it tends to promote goodness*, a little. And when get a bunch of lawful-goods and lawful-evils together and you nudge them all a little toward good through rational discussion (on different axes), that is pretty valuable. Because almost everyone is evil on at least one axis. And such a community needs a policy like "we ask that you be lawful,[follow standards of rational discourse] not that you are good [have gotten object-level questions of policy right]," because it is the only defensible Schelling point.

*If you haven't caught on to how I'm using "law vs chaos" and "good vs evil" axes here by now, this may sound like moral realism, but when I mean by "law" is upholding Yvain-style standards of discourse. What I mean by "good" is not just being moral, but being moral and right given that morality about questions of ethics.