Using the same currency without a common fiscal policy and resources is a disaster. It means currency and interests rates cannot be tuned to each country's economy. It means governments cannot borrow in their own currency (which is cheapest). There is a reason almost all countries have their own currency.

The hope was that the single currency would be soon followed by political union. Without this the Euro has been a disaster, leading to the European debt crisis, and painful austerity as the price of bailout.