This is very hard for all government to use one currency for the whole countries, technically it is impossible so that I think there is no need to make any comment about this from my point of view, there is big difference among their economy some countries are poorest.
Agreed. Every country has different economies depending on how developed it is. If there is only one currency in the world, this would give rise to destruction and chaos from different parts of the world and would also create an unbalance in the system. Having only one currency will only make the poor poorer, and the rich richer due to the availability of the funds to them. Expect war from countries happening and poverty around the world increasing.