The US Dollar is experiencing 10 or 13% inflation (it really is no joke) , so- I can move to Switzerland and enjoy a -0.2% inflation rate with a side of 25% fractional resrve rate, that way I can have confidence that my Swiss Francs will be worth just as much tomorow as they are today, more even.
With a world currency, if my money has 10-13% inflation, I'm stuck with it- I'm screwed.
I don't think it will work. And what will happen with the current currency's? I also find that there will be too much risks and there's a chance that a lot of countries will deny a global currency. The value will also be a problem, how will they determine the value?