World was never one. Boundaries exists in mind and we're accustomed for thousands of years. Forget humans, animals have boundaries too. It is not getting away any soon with one currency. Maybe, force globalization will occur but boundaries will always be there. Don't be so naive. We need change of mind not currency for vision of one world.
That said, its origin is very obvious. VERY OBVIOUS. Lets hope, I'm wrong. Why not leave matter to accept/deny currency acceptance with domain experts? Ask yourself, how much you understand currency? Well, after years of studies, I'm confident about 1-2 percent. Its not just currency we're talking about... but social fabric of society.

Cheers,
I don't say it is one, but it is ideal for it to be one, a single currency will help change mindsets too, and help integrating the world towards being one entity.
Boundaries are always there, within a nation, states are a boundaries, but there are always some things that bind the nation together, a world currency can be a thing that binds the world together.