I don't think Johnson's contention is the final word, but it's an arguable position. And it's not that Freud, Marx, and Einstein necessarily made the world a better place, but that their contributions had more far-reaching effects than than did anyone else's. But it's certainly an open question.

 The study of genetics is interesting on its own, and leads to all sorts of additional interesting questions (and sometimes medical advances.)  There's a fear that studying genetics ends inevitably in a eugenics nightmare. I don't think that's a reasonable fear. Shying away from sensitive inquiry is, at best, the beginning of boredom and, at worst, the basis of censored thought. But you did say you think such discussion should be free and open, and your point is well taken.