> This conflict means that, no matter how well intentioned, policy makers will be feel compelled to devalue their own currencies. It's a race to the bottom, but without a bottom. Alternatives, especially private alternatives readily available to citizens, will increasingly be viewed as the enemy.
The ability to devalue their currency is an essential tool that central banks wield to battle financial crises.
Keynes pointed out that workers don't want to take a pay cut (and employers don't want to offer a pay cut, because they fear causing resentment), so nominal wages are "sticky." When they need to fall to reach price equilibrium, the economy tends to experience mass unemployment instead. It would be very difficult to convince all workers universally to accept a pay cut. The alternative is to devalue the currency so everyone is affected through the currency rather than by trying to propagate policy changes throughout the private sector.
Also, countries can have huge problems when they take on debts in a hard currency which they can't devalue. This is how you get runaway hyperinflation. Their revenues are in the local currency, but their debts are in (usually) USD. If they run low on foreign currency reserves, they can be forced to sell huge amounts of the local currency on the forex market in order to acquire sufficient USD to pay their debts. I can see why France and Germany wouldn't be eager to see yet another USD rise to prominence and leave them disadvantaged in debt financing.
Another talking point which comes up all of the time is the tension introduced when different regions (like Germany and Greece) are forced to use the same currency, even though normally economic forces would cause the weaker region's currency to move against the stronger one's. Devaluation would be fantastic for Greece but it's hobbled by crushingly expensive exports since it's in the Euro monetary union.