"Learn at least one new [programming] language every year. Different languages solve the same problems in different ways. By learning several different approaches, you can help broaden your thinking and avoid getting stuck in a rut."
--- The Pragmatic Programmer
I embraced this philsophy some years ago, and I found it very fruitful. Learning a new programming language is surely simpler than learning a new language: you can write simple programs in a night, and being productive in a week (surely, it takes a lot more to "master" it, henece the "one language for year").It has however the same advatages. Knowing many languages makes you able to speak directly with different people, easing your job; the same is true form programming languages: you can "speak" directly to many different code, which will definitely ease your work!
Another advantage is that you can use the right tool for the right job. This was not perfectly true in the past: if you program was in C and you had to write a little reasoner, it was hard to write it in another language (say Lisp) and then integrate it in the main program: often the choice did not paid off (for performace, reliability, the difficulty to interface the two worlds using strange mechanisms, sometimes even the necessity to write by yourself such interfaces).
If .NET succeeded in making to me a really good impression, its ability to integrate seamlessy different programming languages without imposing you a "standard language" was surely an important point. I found myself re-opening my CS books on functional programming and use ML and Caml again, with my greatest pleasure.
I think my two points can be summoned with this code snippet I found in an interesing blog post
sort  = 
sort (pivot:rest) = sort [y | y <- rest, y < pivot]
++ [pivot] ++
sort [y | y <- rest, y >=pivot]
It's Haskell. If you don't know Haskell, or a similar programming language, you may have the wrong reaction
, put this code in the thrash, and write a 20 lines long version of the algorithm in your "standard language
". Or, if you have learned a different programming language once in your life, you can appreciate the beauty and the simplicity of it.
Of course, Haskell is terrible for other things: but you can compile the code in an assembly, and reuse it from another language. The right tool for the right thing.
Last but not least: like the opening citation said, learning a new language is food for your mind.