Subsidies within governments.
One of the ONLY reasons we form large area governments (e.g., EU, USA, USM, etc) is to facilitate the transfer of wealth from haves to have nots. A shared currency with a central taxation is the vehicle for that transfer. As soon as little subdivisions (New York(USA), South East(UK), Germany(EU)) start accounting and saying "hey, mate! We send more than we get" then they are forgetting that purpose. As we say here, and you've heard elsewhere, within my house, we are all communists (sharers and carers) but at some distance that fades to being something less sharing and caring. For some, that distance is across the street, for others, it is across the planet. When an "across the planet" type sees an "across the street" type doing well, they sometimes get jealous and send in the "boyz with bayonets" to breach that barrier. The relative homogeneity of some countries makes it easier to extend that barrier to the border, but at the same time makes it harder to push it beyond that border. The
social capital benefits of homogeneity, to me, suggests that we should work to increase the perception that we are all "home boyz", rather than encouraging the Balkanization of our societies. Bottom line - don't play the "we send more than we get back card", just be glad that you are better off than those other blokes that need central help. And hope that the centrals don't get so greedy that they drive away the wealth generators. After all, the best parasite is the one that does not kill the host.