[derleth]

The only reason to move from Java to C#, say, is if C# offers a library or tool Java doesn't, which is something external to the language. This is less true for Perl-to-Python moves, but Perl and Python aren't that different, either. You can bring the same conceptual toolkit to all of those mundane languages and get most of the same things done in the same amount of time.

Moving from any of those languages to Haskell or Erlang is going to turn your brain inside-out for a while and when the learning process is done you'll likely approach every other language a bit differently. That is a good reason to learn those languages.