"Ultimately, a stable monetary union requires a stable political Union and at least a degree of fiscal union"

Seems to me that it is also essential to have freedom of movement of workers. One nice thing about the US is that there is absolutely no limitation on moving from one place to another. If you don't like Mississippi, move to NYC. If you don't like NYC, move to North Dakota.

Lots of people won't move because of family issues or fear, but lots of others pick up their bags and relocate at the drop of a hat. Mississippi Delta to Chicago being one obvious example.

NYC to California is another.