Did you know that one language dies every 14 days? I suppose that it is very safe to say that that is a higher rate than people learning new languages.
There are currently 7,000 languages in the world, but many are in danger of extinction. If this is not enough reason for people to go out there and learn languages so that they do not die out, I don't know what a good enough reason is!
Seriously, one does not have to be a linguist to understand that languages encompass more than a means of communication. Languages cover cultures, the way people live and perceive things. They can give us deeper insights and knowledge about people and the environment they live in.
There is actually a project called The Endangered Languages Project, which aims to preserve languages of the world. Even if you cannot take the time to learn one of these endangered languages, you might still be able to help.
On another note, it is not too crazy to think that many languages that are not so popular may find themselves in the endangered language list in the future. It is thus not unthinkable for people of today to try to prevent that from happening by choosing to learn lesser known languages and passing on the knowledge.
Instead of learning how to speak Italian, French, or Spanish, how about Thai, Vietnamese, or Ndebele?