The Chemistry of Diversity"The Downside of Diversity" discusses problematic findings from the research of Robert Putnam. The sociologist discovered that civic participation dropped in more diverse communities, thus limiting "social capital." Putnam himself found the results disturbing, especially as his findings were taken as reason to justify denying rights of immigration, etc. However, his findings also flew in the face of logic: the more diverse communities, which were also larger (mostly cities), tended to be more creative and productive.
I could raise many issues about this: white flight and gentrification as processes that limit the potential of cross-cultural contact. But two things should be noted above all: type of community and how integration works. First, the geographic concentration of cities allows exchanges to occur with greater frequency and rapidity than in small communities, thus multiplying their potential effect. Second, new arrivals don't surrender their identities so much as moderate them continually such that differences are thinned. They slowly fold their way into society. (The "melting pot" was always a bad analogy. No one, especially Anglo-Americans, lives in such a state of flux.)
[ETA:] Perhaps I should word this more strongly: social capital may not be the best measure or explanation of diversity and its benefits.