I guess for me, the problem with world currency is that money which grows exponentially through debt is dangerous. The "wealth" through debt creation scheme rigs itself so obviously in favor of the rich, whether we're talking individuals or countries, and so much worse for those on the bottom. It also encourages the rape of the planet as the increasingly poor have less and less leverage about protecting the planet's assets essential for long term stability. IMHO, the more money is created purely as a matter of real resources or time expenditure, the more checks and balances exist. I don't know that local currencies are the answer, but a revamp that doesn't include debt-based wealth creation is crucial.