The world should have one global currency

A multi-currency system presents the governments a large field for export-import manipulation using the exchange rates: put your currency's exchange rate down and your imports will go up for some time not because your goods got better or cheaper or something - but for purely manipulative reasons. A whole system of risk management is in place in banks and foreign trade companies to respond to an artificial and unnatural risk as the one related to exchange rate fluctuations. A whole industry of forex traders is there to gain profit from not producing something, but simply from speculating on these governmental speculations. (I hate to sound like a primitive marxist physiocrat, but here it's the case when their argumentation sounds reasonable).

A globalized world should indeed have one single currency. Many libertarians, including the Belarusian economist Jaraslau Ramanchuk, propose a return to the gold standard. Even though at the university we were taught that abandon of the gold standard was quite an obvious thing to do (gold isn't liquid enough), afterall one can see the abandonment of the gold standard in 1970s as being politically motivated and driven by the US.

Still, more likely is it that the world will melt into one currency zone following the example of the EU. Regional currencies at first, to merge into one global currency zone after that. But this is something of a science-fiction scenario that could get real in a hundred or two hundred years. A big question is at all whether there would be enough motivation at all to abandon the multi-currency system and all its positive sides (possibility to generate profits from exchange rate fluctuations, possibility to diversify investments etc).