I think functional programming is an useful (and probably fun for some people) exercise to see how far you can go when solving problems with a limited or "pure" set of tools. We got and are steel getting a lot of insight from it. Like forcing a boxer to only box with one hand while the other one is tied behind his back. I'm sure one would have learn quite a few new tricks not to get battered in the ring that way. Things do get weird when people start forgetting the the world is a bigger place and there are a lot of ways do the same thing and the reality doesn't care about purity. That makes some people bitter and they dig down even harder into their limited world and start evangelizing it even harder to signal their smartness. In their bitterness and anger they do not see that functional programming has made it big time. Almost every important language today supports multiple paradigms, including functional.