What's so great about a global currency?  Local currencies are good for some things.  Once upon a time a group of parents created a local currency.  It was used to track babysitting time in the group.  As I recall a 10 point note got you 1 hour of babysitting per kid.  The trouble started when my wife hoarded.  She babysat until she had almost all of the notes.  She complained when more notes were created to keep the system from collapsing.  I suggested she either give some of the notes away as gifts to get them back in circulation or use some for actual babysitting services.  What I missed at that time was suggesting the exchange rate be allowed to float.  If a 10 point note became worth 2 hours or even more of babysitting then the missing hoard didn't hurt anyone.  Eventually we could have replaced a 10 point note with 10 1-point notes, etc., to satisfy the needs of the group.  No one expected to exchange our notes into anything else except babysitting time within our group.  Still, my wife, despite her hoard, needed to buy gas and food, etc.  Our group eventually disbanded as the kids grew up and families moved away, etc.  The notes became artifacts.  One can imagine lots of local currencies with suitable scopes.  Of course it could become tedious to track all of them.  Hmm, maybe a single global currency would be useful.