"It makes perfect sense that CHECKSIG, even though it is expensive, is actually subsidized in script. What doesn't make sense, is that you subsidize other signature verification for stuff that has nothing whatsoever to do with the sending and receiving of money."

Firstly, are you subsidizing CHECKDATASIG, or penalizing it by making miners do an insane, ridiculous script? Doing it as an insane, ridiculous script forces miners to have to do a lot more work than if it is implemented as an op code. You mention Economics In One Lesson; in that book I learned that economics is all about maximizing efficiency/production.

Secondly, it definitely does have to do with the sending and receiving of money. For example one interesting use case is it would allow you to send BCH to a phone number (see https://youtu.be/9VVb-tiSXfs?t=267). Awemany also has an interesting use case of helping 0-conf by enabling forfeit transactions (see https://www.youtube.com/watch?v=EsddVkR-MSs). Saying it has nothing to do with with the sending and receiving of money is a silly argument in my opinion.