While I agree with much of what Peter says, I think that the very pluralism he admires is probably a reflection of diversity of identities rather than the analytic weakness of the notion of civic identity. After all, crusty gadflies often view themselves as such and are emotionally invested in such self identifications, hallmarks of identity.
Pluralism sounds good for democracy, but it's not so clear that empirically it's always beneficial. I'm reading Collier's new book, and he reviews research suggesting that ethnic diversity under conditions of economic hardship--he's focusing on Africa--is associated with the failure of democratic governments. Collier argues that the failure in such countries results from the lack of a shared identification with the nation (or at least that this is a contributing factor).
Collier (2007). The Bottom Billion. Oxford University Press.