There are advantages of using only one currency worldwide. The issue of new Bitcoins is decentralized so no one can complain the power is in hand of one country only like today with USD, but Im not sure if it will be possible to use Bitcoin or other decentralized cryptocurrency for the amount transactions required. The blockchain will be very huge (and just splitting to several cryptocoins have zero advantages - the combined size of blockchains would be the same and you will loose the advantage of one only worldwide currency, compatibility, ease of use)