And if they end up serving no practical use they end up discarded and ignored by scientists and engineers (if maybe not pure mathematicians). The author's point is that there is no such selective pressure on lit crit.
Also, poststructuralist lit crit is not the be all and end all of "the humanities".
Litcrit theory is actually a pretty big commercial success. Derrida et al. sell quite well for academic books, and a Sexy Postmodernism Sex Theory Made Accessible to Undergraduates type course is a reliably decent draw for English departments, who need the enrollment.
Furthermore, if you consume entertainment products with shifting, non-linear narrative like The Simpsons, Coen Brothers movies, or Inception, you're exerting selective pressure in favor of postmodernism.
Practical use ≠ commercial use! Also, I have no idea where you got the idea of "disgarding" research, especially in regards to pure mathematicians. What is "practical" to you?
Also, poststructuralist lit crit is not the be all and end all of "the humanities".
No, it's not, and yet when I deconstructed the author's paper (I crack me up), I found that he made a generalization about all of the humanities following his limited experience with the given topic.
That's like reading a scathing Yelp review of a dodgy restaurant and saying "he hates food!"
Your analogy is like doing meth in a bathtub with a small chimpanzee playing a xylophone to the tune of, "Pachelbel's Canon" while shoving a sharpened pencil into my urethra to wash away the pain of dealing with ridiculous comments on Hacker News.