Diversity, I would say, as a general proposition is valuable, whether it is diversity in ecologies, organizations, or societies. Of course, cultural diversity within a society might function as a centrifugal (etymologically, "fleeing the center") force. Then it becomes a matter of the two sets of countervailing societal forces--centripetal and centrifugal--jousting with each other. Examples of centripetal (unifying) forces include common language, religion, and ideology. Re the latter, in the U.S. we espouse (but do not always practice) a belief in individual rights and freedom. This has been a force for unity.
Probably the best example today of the unification of disparate peoples and cultures is the remarkable achievements of the European Union. If you look at the history of Europe in the 19th "long-century" (1800-1950), it was a history of nearly continuous conflict. For example, Strasbourg, France, changed hands between France and Germany five times beginning in the 1870s. Thus for Europe to economically join forces and construct political peace is one of the extraordinary chapters in global history.
I asked some Polish students a couple years ago whether they self-identified first as Polish or as European. Some of the students claimed a European identity over a national one. I would say part of the new Euro-identity is that they see themselves as a superpower who is "not American" and as an old civilization which is forging a more rational way of living. However, Europe contains within itself some gathering centrifugal forces, especially that of rapidly increasing immigrant populations who are not well integrated into European societies. That will make for interesting watching over the next few decades.
In the U.S. the national discussion concerning diversity continues with the present presidential election season. Has this not been perhaps the main topic of national civic discourse since the beginning of the American experiment, the American discussion?