This point - about the rationale for the nation state - is key to understanding the resistance to international migration. We need to address head-on the trade-off between diversity and national identity. Sen's book Identity and Violence has an interesting discussion of a solitarist approach to identity (national identity) vs a pluralist approach where an individual has many identities, and John Gray, in his review of Sen's book (see http://www.theguardian.com/books/2006/aug/05/highereducation.shopping1), mentions examples of how nations are built after much bloodshed and suppression of some forms of identities. This discussion then leads to the definition and efficiency of national borders: the border is not the same for goods as it is for services, and it is not the same for migration, and it is not the same for you as for me. And we are definitely getting more comfortable with different borders for different things and even "less" border for all things than, say, before 1990. There is a regression to erecting borders arising from the recent financial crisis-induced paranoia, but I hope this paranoia is temporary. Time has come to open the case (or lack of it) for the nation state.