How the world would be if there is no $,no €,no £,no ¥,no rupees but only a "SINGLE CURRENCY"?

euro is used as a single currency in EU countries...in the same way how the world would be if all UN countries use a single currency apart from dollars,pounds,euros,yen and indian rupees....?? it looks like a stupid and meaningless question but i have to clear my doubts...thanks in advance:)