This article isn't the full picture of Caplan's view. I haven't read his book, but here's a timestamped talk [0] at The Heritage Foundation where he addresses this argument. To summarize, he says that he loves "useless" knowledge but that it should be enriching (points to amount of busywork), and that it requires that students are willing participants, which he argues that many of them are not.
Caplan says that the book has an entire chapter dedicated to refuting this argument, which I can't speak to.