Practical reasons for learning some functional languages:
- Lift Web Framework: http://liftweb.net/
- Java interoperability
- New O'Reilly book, "Real-World Haskell": http://www.realworldhaskell.org/blog/
- Most modern functional language in semi-mainstream use.
- Great books: "Lisp in Small Pieces", "The Scheme Programming Language" (by Kent Dybvig), and "The Structure and Interpretation of Computer Programs".
- Good, cross-platform IDE: DrScheme
- Ability to leverage knowledge for Arc (Paul Graham's language which will almost certainly have a large following due to his popularity)