Currencies do not necessarily have to have a total order (you don't like that, do you) or hierarchy.
Bingo!
National currencies were/are the citizen's unit-of-account because it is very costly store your cash flow in a unit-of-account which is not the predominant unit-of-exchange.
Now that we are transitioning to B2C and P2P (C2C) globalized commerce, we need in some areas (especially intangible Internet commerce) a globalized unit-of-account and exchange.
Read that second sentence again and understand the profound implications. The coming moat is no longer national boundaries but tangible versus intangible (Industrial Age versus Knowledge Age) commerce. Also make sure you read my essay in the Economic Devastation thread.