This is the most important point which many are missing. Having 50 different currencies would be a nightmare and a huge drag on commerce. The common currency is one of the main points which has made the US economy what it is. Having a common currency is a huge benefit for commerce while having to trade is flcutuating currencies is a huge drag. What a state gives up by adopting a common currency with others is more than made up by the benefits of having that common currency and European countries have see this.
The fact is the world is already headed to having a few major currencies much faster than we think. Many countries have already given up their monetary independence in exchange for the benefits of having a common currency. countries which use the US dollar or the Euro or which have their currencies directly tied to one of them:
US dollar: Liberia, Micronesia, Palau, Puerto Rico, Panama, Ecuador, El Salvador, Guatemala, Hong Kong, Bosnia, Bulgaria, Estonia, Lituania, Yibuti, Kiibati, Lesoto, Namibia, Swaziland.
Eurozone: Germany, Austria, Belgium, Spain, Finland, France, Greece, Ireland, Italy, Luxemburg, Netherlands, Portugal, Andorra, Vatican, Monaco, San Marino. African Finacial community Franc (tied to the Euro): Benin, Burkina Faso, Ivory Coast, Guinea-Bissau, Mali, Niger, Senegal, Togo.
Then you have other monetary unions in the Caribbean and elsewhere.
Many countries have gladly given up their monetary independence because they had more to gain than to lose by it and the trend will continue.