Language is a reflection of speaker's model of the world, a different unique perspective on the same world we all live in and hence can provide diverse solutions to the same problems we face. Diversity in thought process makes it faster to decipher that beautiful Nature. So it will be sad if these languages die. Of course, this is not the only reason, but I think is one of the important reasons.

I speak three languages, I think in two languages. I go on different paths when thinking in different languages when trying to solve a geometry problem.