The second reason has to do with linguistics, when people from different groups live together the pressure to pick up additional languages or words from those languages increases...for the young this is actually beneficial for the mind as multiple language cognition has been found to benefit individuals who've gained the skills as adults. It also enables those children to again be culturally exposed to those different languages and makes them less likely to acquire strong biases against the different groups.
Another reason comes from genetics, two populations that share genes create a more fit third population that shares traits from the original two. If in isolation groups have created specific mutations that tend to be carried within the group as detrimental or positive traits, mixing of the populations enables detrimental traits to thin out (as the percentage of their propagation goes down as in group individuals mate with out group individuals that do not carry the trait) and positive traits to propagate (as they are positive because they confer some survival advantage which likely extends beyond the originating geographic origin point).
The final reason in my view is that this type of interaction is actually beneficial for humanity, dare I say essential over the next few decades to allow the last fires of nationalism that burn in the hearts of many parts of the world to finally die out without conflagration into the conflicts we have going on currently derived from such heat. Chechnya...Kosovo...Darfur....ethnic/linguistic conflict has been the bread and butter of human violence for as long as humans could hurl a stone across a river and discover a people there that spoke differently, looked differently and prayed to different Gods. This last reason is secretly my prime reason for wanting to release numeroom.com eventually (once I launch it) to the consumer masses to enable a true language agnostic communication platform (not separate islands of language on one service) that will be used across devices to just make the idea of different language on such services irrelevant...whatever language you speak that is what you should see...weather or not those you are communicating with actually are typing in that language. Small scale systems for doing this have been built but no geographically scalable implementation with an "implicit" translation existed until I created one in numeroom.com (troubles with licensing translation tech. from google and other dictionary providers aside!)
Mixing humanity is key to our survival in this difficult time as we kill the oceans at rates unimagined 5 years ago, as we choke the skies with coal and as we denude the land of it's oxygen producing arboreal protections. Let us hope that we can embrace this diversity of experiences, lives, people and ideas to grow beyond the self destructive ways we currently exhibit.