I'd rather ask the question without the word "sometimes". Because what people do is use that word "sometimes" as a rationalization. "We'll only use the Dark Arts in the short term, in the run-up to the Singularity." The notion is that once everybody becomes rational, we can stop using them.

I'm skeptical that will happen. As we become more complex reasoners, we will develop new bugs and weaknesses in our reasoning for more-sophisticated dark artists to exploit. And we will have more complicated disagreements with each other, with higher stakes; so we will keep justifying the use of the Dark Arts.