Its all about trade between countries. Tourism and cross border travel is a
tiny part of the economic benefit.
Countries with very interlined economies - such as Poland and Germany - have very good reason to share a currency. The periphery such as Greece, Ireland, Spain and Italy should not be in the Euro.
But for Poland, Holland and even France its a no brainer.. Poland was part of Germany's infrastructure until 1945.
You never hear about a trade deficit between states in the USA because it just doesn't happen, and it can't happen in Europe any more either.
It impossible that every state has a 100% balance of trade with every other state, so thats a silly comment (unless you want to fool yourself that Alabama produces and exports the same as -say- New York or Californian?). The USA is, like the UK, a
transfer union, money goes from poor states and regions and this is accepted.
The same transfer union has to exist in Europe.