First of all, using dark arts does not imply you have to tell outright lies.
It does, in a very real way: if you say "You should believe X because Y", and Y is not actually a good reason to believe X, you have spoken a falsehood, whether Y is a lie about external reality or just some rhetorical trick.
Secondly, you say "if a person ends up with better ideas but all the same biases, their heads can later just as easily be filled with whole new sets of bad ideas by other Dark Arts practitioners." When the alternative is that they only had bad ideas in their head, this is a still a win.
I am not convinced of this. You are right in a strictly utilitarian sense — it is better for someone to have good ideas for bad reasons than to have bad ideas for bad reasons — but in most cases, it's a false dilemma. At most, using the Dark Arts is only justified if someone is absolutely intractable in attempts to debias them. Using them too soon could amount to a missed opportunity — causing us to declare victory and move on too quickly, and/or causing them to be even less open to a more fundamental debiasing afterwards. Let's say we convince a Christian to believe in evolution by arguing as though it can be reconciled with a day-age reading of Genesis. This is a bit better than them being a literalist young-earth creationist, but they have not become more rational. And if you convince them of evolution thusly and then try to convince them that their religious epistemology is wrong altogether, I can imagine them saying "Come on, you convinced me to believe in evolution, what more would you want from me?" or "Liar! You said believing in evolution wouldn't be a slippery slope to atheism!" or "What, so I believed that science was incompatible with my religion, and you convinced me they're compatible after all, but now you've switched to arguing that they aren't?". If you want to make someone question their deepest and most cherished beliefs, they are likely to take you even less seriously if you previously convinced them of a lesser point by acting like those beliefs could be true.
(That is a hypothesis, supported only by some personal experience and intuition. It can probably be tested; until then, I invite anyone with personal experience or other thoughts on this point, whether supporting or opposing it, to share them.)
And your example is the minimum win possible. What if we used dark arts to help someone remove a cognitive bias? Is it now justified?
I'm not quite sure what that would look like. Do you think you could formulate an example?
I might be persuaded that this is justified. It would be something like writing a computer virus whose purpose is to patch the security holes it uses to get in in the first place. But before making any judgment I'd still like an example of how it could work in this context.
Third, PZ Myer chose a very effective persuasion strategy, The Admirable Admission Pitch. However, one case where someone was effective sans-dark arts hardly proves the sans-dark arts approach is optimal in general.
You write that "it can disarm opponents, and portray you as a paragon of reasonableness and open-mindedness." I seriously doubt that his usual opponents will find him more reasonable, open-minded, or respectable on this basis (at least in any lasting way), and I expect he knows that perfectly well.
Still, it was just one example, and I could be wrong. I'll retract that argument. You get to the deeper point when you question whether "the sans-dark arts approach is optimal in general". Optimal for what? Perhaps we don't even care about the same underlying goals. If someone cares, for instance, about promoting belief in evolution, then the Dark Arts might help. If someone cares about promoting atheism, the Dark Arts might help. But since we're talking here about promoting rationality itself, using the Dark Arts is promoting the very thing we are trying to destroy. It could cause all manners of misconceptions: 1) To the public: that rationalism is akin to sophism or a religion, and that we are not to be trusted; don't listen to us or else we might trick you into changing your mind. 2) To the people we persuade: that we're authorities to be trusted and accepted, not ordinary people setting a good example to follow. (Or, even worse: they become rational enough to notice how they were manipulated, but then they do follow the example we were setting, concluding that the Dark Arts are an okay thing to use in general.) 3) To people we fail to persuade: that we're dangerous liars who must be stopped.
It's simply the opposite of the kind of epistemology we want to promote. I think the worst part would be the bad example it sets.
Video of killing a cognitive bias with dark arts: http://www.youtube.com/watch?v=haP7Ys9ocTk
(Also illustrates Bongo's first bullet point in a comment above)
The product of Less Wrong is truth. However, there seems to be a reluctance of the personality types here - myself included - to sell that product. Here's my evidence:
We actually label many highly effective persuasive strategies that can be used to market our true ideas as "dark arts". What's the justification for this negative branding? A necessary evil is not evil. Even if - and this is a huge if - our future utopia is free of dark arts, that's not the world we live in today. Choosing not to use them is analogous to a peacenik wanting to rid the world of violence by suggesting that police not use weapons.
We treat our dislike of dark arts as if it's a simple corollary of the axiom of the virtue of truth. Does this mean we assume the ends (more people believe the truth) doesn't justify the means (persuasion to the truth via exploiting cognitive biases)? Or are we just worried about being hypocrites? Whatever the reason, such an impactful assumption deserves an explanation. Speaking practically, the successful practice of dark arts requires the psychological skill of switching hats, to use Edward de Bono's terminology. While posting on Less Wrong, we can avoid and are in fact praised for avoiding dark arts, but we need to switch up in other environments, and that's difficult. Frankly, we're not great at it, and it's very tempting to externalize the problem and say "the art is bad" rather than "we're bad at the art".
Our distaste for rhetorical tactics, both aesthetically and morally, profoundly affects the way we communicate. That distaste is tightly coupled with the mental habit of always interpreting the value of what is said purely for its informational content, logical consistency, and insight. I'm basing the following question on my own introspection, but I wonder if this almost religiously entrenched mental habit could make us blind to the value of the art of persuasion? Let's imagine for a moment, the most convincing paragraph ever written. It was truly a world-wonder of persuasion - it converted fundamentalist Christians into atheists, suicide bombers into diplomats, and Ann Coulter-4-President supporters into Less Wrong sycophants. What would your reaction to the paragraph be? Would you "up-vote" this work of genius? No way. We'd be competing to tell the fundamentalist Christian that there were at least three argument fallacies in the first sentence, we'd explain to the suicide bomber that the rhetoric could be used equally well to justify blowing us all up right now, and for completeness we'd give the Ann Coulter supporter a brief overview of Bayesianism.