Money enables the trade of goods and services. It acts as a store of past value of things that have been sold while you are waiting to buy something else. In reality, it does not matter what form the store takes as long as everyone accepts its value. Maintaining more than 1 currency is inefficient. Maintaining accounts and prices in multiple currencies, being subject to exchange rates changes and exchange costs, all add to commercial business planning uncertainty and cost.
We should use a single currency.
Risk: Tying a currency to a geographic area ensures the wealth cannot actually leave that area. Even if the wealth has distant owners, it must still circulate in that micro economy. A global single currency may strip some areas of wealth and accumulate it in others. Mitigation: A global system must ensure this is mitigated by common tax, minimum wage, wealth tax and global management of social services and investment in infrastructure.