Now suppose I were able to persuade him that something like evolution by natural selection took place, even if I did so by some fundamentally dishonest means (by citing certain Bible verses, say), and in a way that led to no increase in his overall ability to think rationally about anything. If the result were that he kept his damn mouth shut about bogus scientific controversies in the presence of impressionable children, thereby not impeding their future progress as rationalists, the broad cause of rationality might well have been helped along.

If, on the other hand, he told these impressionable children that evolution was true because these verses in the bible say so, the broad cause of rationality has been hurt.

(There may be a less convenient world where this is not a concern, but I want to make the point that you might not accurately predict the effects of effective persuasion through the dark arts, and their use might not be as contained as you would hope.)