To be short and simple, economic "value" is the price of goods and services. BTC is not backed by any good or service. Exemplifying, as some can say miners are providing services, BTC would be backed by a service if 1 BTC could be redeemed by 1 doctor visit. On the other hand, the service by miners is not value, it is operational cost.
Please understand I'm not against virtual currencies, I think they can be very useful in the future.
Fiat is not backed by anything at all. Not only that, banks lend out more than ten times the amount they nominally have. I'm close to just writing you off as a troll at this point.
Actually, it is backed by all the goods and services which are denominated in that fiat. The price suggested for an exchange is a confirmation of the trust the producer has in the currency. When you accept payment for your work, you do the same. If you think thoroughly about this, you will inevitably come to a conclusion that such a system doesn't differ very much from a money system where currency is backed up by some hard asset like gold...
Like many on this forum you seem not to understand how FRB actually works. If you think otherwise, please explain how a bank can have more liabilities than assets?
So all that's needed is for people to accept payment in bitcoin and it magically becomes "real"? Cool.