If we use a one currency for whole world is also not a good idea which has been repeatedly avoided by developed world nation it can take steps to deflate it currency making it cheaper and therefore make it export more appealing to other countries with only one global currency having single currency is not enough to raise a question you also have to state of countries have the right to print their own currency if people of one country can go and buy or sell in another country without any cess or taxes people are free to invest in any country.