Actually knowledge of of any those is a proxy for the general quality of those developers. They could end up writing C++ but the fact that they went out and sought to learn language they thought was beautiful or had a different paradigm tells something about them.
I get that it is possible to generalize public opinions and say that something is "objectively" beautiful, or ugly. But I don't get why people familiar with those three languages always praise their functional purity, as if imperative languages are barbaric. How come there aren't ugly functional languages?
I'll talk about Erlang for example. Functional purity is objectively beautiful because it solves or minimizes a large number of typical problems. These problems such as large mutable states, large mutable state modifications during high levels of concurrency. A good functional language, using closures can mimic other patterns while having a very small core set of rules.
Purity doesn't mean moral superiority (although it sounds like) purity means referential purity for example, where the function when called no matter how many times with the same argument returns the same result. That is beautiful because it allows for interesting compiler optimizations, good for testing. Other looser explanations of purity are -- confinement of mutable state (this could mean monads in Haskell modifications to shared global state), immutable data structures in Erlang or Clojure.
Now that said these are all tools. As I mentioned, in practice, a lot of these people in their day job will end up using Java or C++. But the fact that they decided and managed to learn a new paradigm is what is the key.
Heck it could have been data-flow programming or logic programming (Prolog is awesome too, especially when mixed with constraint satisfaction).
Yet another way of putting it, familiarity with these things point to a level of passion and sets someone apart, that is quite desirable. Now, yes, there is a self-referential quality to it all, the more we think functional programming is a proxy for developer quality, the more people will do tutorials just to put it on their resume. Well, then the next fashion will be something else -- quantum algorithms perhaps, who knows.
I am sure those high caliber gurus will have no problem learning javascript overnight. That wll be excellent addition to their toolbx for that rare case when their internet enabled fridge gets hijacked
