There's an old saying that goes: "when the only tool you have is a hammer, all your problems look like nails". If all you learned was imperitave programming, you'd tend to approach all your problems with a procedural mindset. There are some problems which are easier to approach from a functional point of view (map traversal and processing come to mind). Some areas of study tend to use functional programming over imperitave (AI, for example). You never know what kinds of problems you will have to solve, and you would not be well served with a toolbox that only had a hammer in it.
Learning unusual computer languages have a lot in common with learning new spoken languages (e.g. French, Japanese, Hindi).
(a) seeing abstract patterns in language gives one a broader view. I still remember the first time a professor told the class we would use a language that didn't have a "for/while loop" (APL, Lisp, etc) -- and yet it was a full Turing machine.
(b) not only does one have more tools, but one has more concepts -- some _ideas_ are very hard to express in one language, and yet are easier in another. _But_ this understanding can help you use any language in new ways. A "function object" is a powerful tool in Java, and yet has ties to functional languages. Also true for the current debate on closures. Learning Ruby can make you a better Java developer.