Learning Haskell isn't beyond any programmers though, and is likely to pay off long-term. I agree that other fringe languages might have less payoff though - Erlang/elixir, Clojure, F#/OCaml, Idris/Coq/Agda (although who would write production code in these?) are all more likely to impart fundamental understandings than e.g. Coffeescript, Wisp, etc.
Learning kung fu or the violin isn't beyond most people either, but I'd rather not have that be the prerequisite for being able to be productive on my team.
Most companies don't prioritize paying a potentially highly compensated worker to learn an obscure language when the job can just as effectively be solved using standard skills and languages that are much easier to acquire(hire).
Also, despite the passionate defence that many functional-first language advocates give, I have never seen any conclusive evidence that these languages, in general, produce better results than the standard suspects.
Having a team member create production code in a language nobody else on the team understands is a risky decision. That applies for anything, software, systems, infrastructure... if one developer is allowed to sit on something, what happens when the dev leaves? Or dies? And the thing he or she worked on had a non-trivial learning curve?
Choosing something like Haskell or anything else niche should be a group decision. Even when it comes to prototyping! "Guys, I'm going to prototype this .NET code in F#. Who wants to join?", instead of a developer sitting in a corner applicating his functors while others around him do something arguably simpler. It's incredibly short-sighted!
This is not aimed at you though, since I'm not sure if this was your case, I'm only guessing based on your post.
