If we will use one currency only, the value of money around the world will be the same.  If that happen, more people do not have to go to other countries to work. Meaning, long distance relationships will be eliminated.  There will be no inflation and deflation of currencies because all currencies are equal.  The idea of having centralized currencies is interesting but it must undergo in careful and properly discussions and deliberations of all countries and clear set of rules and regulations must be formulated before launching it.