A single currency would require a global consolidation of wealth. That would make a currency useless.
You see, here's the thing. A currency is a representative of how much gold a country will vouch for, compared to other countries.
To have a single currency, you'd have to have all of the countries combine all of their gold. That would mean that there would be no competition between each country, so the currency would be useless.
Every country would have equal buying power, so there would be competition.
No competition, means capitalism doesn't work. So you'd head down the path of socialism and communism.
Communism does away with all of the currency.
A one world currency would be amazing because it'd mean you're one step away from a communist paradise.
That said, the one world currency would then be useless.
Cheated the way from fringe to elite. Clique of stylists, rounded illogic skipping a beat to a dead cert. By lheaving charges and bursting the abscess, with a forked toungue, bloated with courage and spewing self-importance. Drop your sights, aim lower, leave umblemished those with real power.