The problem I see with this single currency, apart from what it says @shainemata
It is that to be a single currency most states will disagree with being somewhat decentralized. And this being the reality, who could assume what that single world currency could be?
We well know what an institution like the International Monetary Fund or a central world bank can do in terms of economy, I don't know, it is an idea that could bring solutions to different problems but add others.
But it sounds good.