Different physical appearances may have profound psychological effects (not always conscious) that effect social interactions and the formation of relationships. There are also significant personality differences between different races of people that be a hindrance to social interactions, not unlike the general personality differences between the different genders.
I think that increasing diversity is going to create much more social isolation, and this will not be good for society. Immediately, the effects of this isolation will be felt most by the minority children themselves, but in the future, if the main ethnic group itself eventually becomes a minority everyone will feel these negetive effects. Just imagine living in another country where nearly everyone else is from a different race, and it just becomes impractical to meet anyone of your own race. While cultural differences, language, and potentially income levels, may play a role, I believe it is much more than that.
At the very least, increasing diversity is going to create pressing social challenges. Should immigration policy be aimed at trying to minimise ethnic diversity, to protect the best interests of the future generations and the society that they will grow up in?
Psychlogical effects of racially diverse facial appearances
Social isolation can lead to debillitating childhood experiences. http://www.youtube.com/watch?v=Uf4_KYKWieE