I think you could question people's desire to save endangered species in much the same way. Ultimately I think it comes down to the fact that languages (and species) are seen as intrinsically valuable to some extent and the world is just a sadder place without that richness. Language isn't the whole of culture but it's a big part of it, and it seems to me that if culture is worth preservation then so is language. I can't really deny that from an ultra-rational perspective it'd probably be more efficient, in some sense, to have a world monoculture; heck, the world might even be a much more peaceful place---but I still just don't want it.