I don't really see your point about something "of value" backing a currency. Gold has backed currencies, indeed gold has been USED as currency, but even gold is valuable only because there is sufficient consensus for it to have such status. Bitcoin has a lot in common with gold: it is rare, it is divisible into large and small units, it is durable, it is mobile. If there is sufficient consensus giving something valuable status, isn't that enough to make it tradable for other things and as such, a currency?
The consensus of gold being valuable has lasted over 5,000 years. The consensus of a bitcoin being worth more than a hamburger has only been with us for a few months. A week or so ago the consensus said 20 hamburgers and now today the consensus say 10 hamburgers. The consensus can do strange things when trying to assign value to bits on a computer.
Bitcoins are valuable because they are decentralized. Backing would require trusting a central party's promise to trade something for Bitcoins. If Bitcoins were backed by something, they wouldn't be decentralized. Therefore, Bitcoins are valuable because they are not backed at all.
Bitcoins can be backed and still be decentralized. If the backing worked your better off with it than without it. If the backing failed it would be the same as if you didn't have it.
As a disclaimer I would like to mention that I believe bitcoins would be more valuable as a currency if they were backed by something of physical value like gold, silver, rice, oil, gasoline, or perhaps even the kilowatt-hour.
Those things are themselves "backed" by the conjecture that they exist in limited quantities and require a lot of work to produce.
Bitcoin solves the currency issue by generalizing this into pure math. Those who grok it see them as the best currency backing you can achieve.
Would you rather have a coin that promised Someone was given this coin as proof they mowed a yard or a coin that promised I'll mow your yard when you give me this coin. A currency should promise something meaningful to the holder of the currency so it is important to look at what that promise is.