[Miisa]

The article isn't talking about having a global currency, but rather NOT having the US dollar as the reserve currency that all other currencies are tied to. So actually sort of the opposite: that way economic fluctuations in the US wouldn't bring the global economy up or down.

The problem with unified currencies lies in the situation where one or some countries do something that undermines the currency. It means the damage is spread out over more countries' economies, but the people in those countries might not be too happy about their economies being dragged down, especially if the offending country is a major economic force (though there is little complaint or even comment when smaller economies are boosted upwards by larger ones). That was the case a few years ago with the Euro area, when (IIRC) Germany was doing stuff the others disapproved of.