Think first of clothes. Then, uniforms. Then, decide: do you or do you not like variety?
Come to think of it, the composition of distinct races in the early days was more pronounced than today. People stay in their respective places and travel was restricted to shorter distances. Thus, culture exchanges were limited.
When the modes of transports improved, travel over great distances became a reality. Colonization opened the way to move people of another race to a new surrounding populated by another. Naturally, clashes about differences in just about anything began.
I believe the mixture of the races is beneficial in the long run. With no pure bloods (impossible I think), no one could stay in a pedestal and proclaim superiority over others.
Can we focus more on uplifting the lives of the less fortunate in this world? I believe that is more of an ideal human quest than all the petty quarrels we have at the moment.