Jay Matthews asks this question in a rather ambivalent discussion.
I've noticed four main arguments for everyone learning another language:
1. Kids need to learn a new language for exposure to other cultures. This will reduce the amount of racism in society.
This is a good point--though some people are probably uncomfortable with this kind of social engineering. What I don't understand is why the classes, then, are language-centric and not culture-centric. Why not have a class that survey's other cultures, focused on food, music, dance, and sports?
2. Learning a new language improves kid's ability to speak and write well in English.
This is particularly prevalent when people argue everyone should learn Latin or Greek. What I wonder is if there is credible evidence learning Latin helps with vocabulary and clarity more than studying vocabulary directly and reading Orwell?
3. Learning about new languages is interesting (to some people) so everyone should be forced to do it.
This is just insane. But you'll hear it in every debate. Perhaps sabermetrics should be a required part of the high school curriculum?
4. It's not that everyone should learn a language, it's that everyone should learn Spanish.
This makes some sense to the extent we want to be a bi-lingual country. But I can't see many benefits from that, and it's possible a two-language equilibrium hurts everyone's welfare in the long run.