At least for political reasons it is too early to talk about one single global currency. And economists can mention plenty of economical reasons as well. Anyway, it is a bold mind, which is deserving attention.
There aren't particularly "bad" economic reasons not to have one single currency, it's more that it's a utopian notion because:
- nation states don't want to surrender their sovereignty by giving up their currency(that's why for example it's not done by all EU countries, despite them banding together to compete economically with USA and China due to Europe's decline)
- private interests(i.e. national business elites, volume of international currency exchange and speculation, "hoarded" private assets in various currencies)
- the world is still divided by nation states, not a global central government
The idea of a world currency is not new: there would no longer be currency risk in international trade but, short of accelerating integration, most countries are not likely to give up their ability to regulate national economies in the foreseeable future.