Among the supporting arguments for the forthcoming appearance of new currencies managed by private entities are:
Private entities are appearing which begin to wield more power than many states, and which are apparently well intentioned : Gates foundation, Google, etc.
Google's motto, for instance, is "don't be evil", and the firm is in the course of gaining sufficient power to issue a new private currency more reliable than many official ones. Google is gathering experience is the management of financial and monetary means: online payment system, adsense and google ads, controlling information, etc. We may soon witness the discreet appearance of means of managing exchanges outside the realm of official currencies, with points, tallies, or "googlars" (prediction made in July 2008).
Of course, the evolution of exchange rates -and what will drive them - between various private currencies will raise whole new and interesting problems.
History, though, teaches that most world tragedies have at their origin a mix of "good intentions" + "unlimited power to put them into applications" (church, colonialism, communism, etc.)...