All nations would certainly benefit since there would no longer be currency risk in international trade. Traders would no longer fear for currency fluctuations.
It is true that having only one accepted currency for everyone in the world has its perks we have to look at it realistically.
First of all which countries would be willing to give up their own currency? The poorer countries will of course benefit even though they would still need to do lots of work. If the world sets up a union with all the countries, who would be its leaders and founders?
Nations all around the world value their own identity which is represented by their currency. You might think it is just for economic reasons but a lot of countries would not be so willing to give up their currency for a lot of other reasons.
It might just be impossible in my opinion or at least for now.