I wouldn't quite agree that tax requisition is "the thing" that gives a paper currency value, though of course it is one. (I mean, lots of people/companies hold USD, EUR, CHF, JPY, etc., without ever needing to pay taxes in those currencies.) But my main point is that it's not commodity backing that matters, and that currencies don't need to have "intrinsic value" in some other use than as currencies.

In any case, I find it kind of amusing how much incensed criticism Bitcoin gets simply for being a currency that exists. It's like, imagine people wrote thousands of angry, scathing articles and comments all over the internet about the Azerbaijani manat or something.