Most reasonable paper currencies are backed by the fact that millions of people need to pay their taxes in that currency. That's generally what gives fiat money value.
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.
I don't think vkou was claiming that you yourself need to pay taxes in the currency yourself. Just that the fact that there's workers out there in another country who needs to pay their taxes in their local currency and will therefore need to charge for their work in that currency or trade for it. Either option will lead to some predictable demand that you can depend on whether you pay taxes in the country in question or not.