[util]

It seems like this in particular applies to most government-issued currencies as well (except that they have very good marketing and market share). Each currency itself has no intrinsic value and depends on "marketing" (reputation of the government minting it) and market share (number of places accepting it). If people lose faith that a government will do a good job managing its currency or even think that the government will disappear soon, the worth of the currency (in terms of what people will give you for it) will probably fall.

[vladd]

A government collects taxes from all the businesses and people in a given country, which is huge in terms of being able to provide value. Bitcoin has no such thing.

[util]

Having access to more wealth gives people more faith that the government will be around a long time, will not do crazy things with the currency. That's not the same thing as the currency having more intrinsic value. (Also, it would not seem to make sense to talk about backing a currency with taxes denominated in that same currency, would it? But I'm unclear what you meant by "provide value".)

(What I've heard about bitcoin makes me a bit skeptical, but I want to be skeptical for the right reasons.)