At the risk of sounding inflammatory... I would be willing to believe that purely functional languages are the "one true way" if there were even the slightest bit of empirical evidence to support it. But where are all the successful large scale projects built with Haskell (or Clojure or whatever)? The biggest FP project I was aware of was Twitter using Scala and they ended up completely backing away from it. Yikes. T…
Now what percentage of the non-functional ones ends up becoming successful enough for you to hear about them and know what they were built with? Apply that percentage to the functional projects, you might just be left with naught.
I have a feeling that if there's one thing more rare than functional programs, it's highly successful ones.