This is a very interesting question. The sovereign nations issue their own currencies and are taken for granted by many people, but few consider whether this is the economically optimal option.
Will ,this idea of one single currency only use to the whole world is only an elusion,because its never been happen,this is only a drastic dreams,because if this happen the whole country will have a one government,and one currency, the people or the law maker always arguing each other,because some are obstructing the opinion while the other are not,thats why this speculation,would never been happen,because many nation will affected.