Stephen Walt spells out the advantages of tolerance, openness, and cosmopolitanism from the realist respective (thanks to Jon Rauch for the pointer). He goes to great pains to point out that he is talking about cosmopolitan openness not just ethnic assimilation.

[T]he pressures of international competition give an advantage to any society that can "cream" some of the smartest and/or hardest working people from all over the world. How? By making that society an attractive place to live and work, mostly by creating an atmosphere of equality and toleration. ...
And note that this argument isn't just about ethnic assimilation. In effect, what I'm suggesting is that from a realist perspective, there is a strong case for "small-l" liberal toleration. All else equal, societies that establish strong norms and institutions that protect individual rights and freedoms (including those governing sexual preference, I might add) will become attractive destinations for a wider array of potential citizens than societies that try to maintain a high degree of uniformity. And when you can choose from a bigger talent pool, over time you're going to do better.