That is a great question. A peek at Google show China is pressing for a world currency. I did not read up on it yet. I do know exchanging currency is easy these days one only needs to use PayPal or its like competitors based in other world regions. For instance I purchase at Amazon.UK in dollars all the time, which is exchanged to Pounds. Plus, when I send money to that person or him/her to me with PayPal we can specify the currency used. The exchange and fee is determined by PayPal.
So, to me that is one step closer to the possibility for a single world currency. What might ponder BitCoin? I do not know how well that performs for a model or as a trend having usefulness.
One thing using a world currency contrast an exchange would be differences of economy internal a nation contrast another. Those fluctuations may not be accounted for since the world currency would be stable or I would guess so. Probably the Euro will be studied extensively as a model regarding European economies.