I would echo this advice as well. As a mainly PHP developer I have learned a lot of cool ideas and techniques from Ruby on Rails that I have brought into my PHP code.

For example, the idea of the "flash" and how to properly do MVC.

Just learning another language (programming or not) opens your mind to new ideas and ways of looking at things. If those aren't reasons enough, then what about increasing your work skills and marketability?

I completely agree with this. Up to a couple weeks ago, I had only programmed in .NET and php. I got bored one day, and decided to learn python. I loved it right away, and now I'm learning Django and building my first web app in python. You can ask other people about which they think is better, but you're going to always get different opinions, so the best thing to do is try to figure it out yourself.