Of course immigrants and different languages increase diversity. But it's also true that other people from within the US are critical to developing a balanced view of the world. A black person is going to have a very different experience that I will as a white dude, and we are all poorer for it when we don't know people with those experiences. The same goes for the white farmer who grew up on a ranch in Nebraska, I would love to know more of those people, they see the world differently than I do.