Efficient market theory is based on the fact that when you leave people to their own devices, they tend to do a far better job and be better off than if you try to help them. Basically everyone agrees on that.
So why do we accept efficient market theory for basically everything, yet when it comes to currency someone has decided that we need elaborate schemes of variable government decreed micromanagement in order for things to work?
In terms of keeping things simple and letting the market decide, the perfect currency would be global, unable to be printed or manipulated, of a fixed amount, so that external interference is impossible, and people knew exactly what they were dealing with at all times. The closest we have is gold (and maybe bitcoin).