[citizenparker]

I think learning a language that may not be applicable at work has two key benefits:

1. I learn it "enough" to know what it's good at, what it's bad at, and to have enough hand-written code to pick it back up pretty quickly (particularly if you set up a test-driven learning environment)

2. Even when I forget the language's particulars, it changes the way I see other programming languages. For instance, while I don't use Ruby in my day job yet, I still think of things I can do with method_missing and how I might approximate that same power and flexibility in my work where appropriate. In short, learning languages helps me program "into a language" rather than "in a language", to borrow Steve McConnell's terminology.