I am of the opinion that one can get better at a language by learning another. You learn the limitations of the languages, and end up pushing your own limitations.

It makes it easier to communicate with other programmers as well (e.g. you can't communicate with the world if you only know English), which leads to expansion of the languages themselves. I can't count how many times that I've challenged my rudimentary knowledge of Perl with what I know in VB. I know that VB can do certain things, and I search out the way to do them in Perl. Without knowing that I can do certain functions in VB, I might not even try to search out those same functions in Perl (or vice versa).

Don't get me wrong, I am quite infatuated with Perl, but I think that learning other languages is highly beneficial. Those that reject other languages wholesale haven't spent enough time with them. It's almost like seeing "Microsoft sucks" posted by UNIX folk who haven't ever seen an NT box (don't flame me on this - I have no grudge against UNIX).

Converting between languages, or thinking in several languages will make your brain hurt, but no pain, no gain.

-OzzyOsbourne