Currency backed by gold, not an illusion. Currency backed by oil, not an illusion. In the future, perhaps we can have currency backed by energy. Aka, energy options, and I don't mean backed by spent energy, like BitCoins.
Bitcoins are not spent energy. Their value comes from the amount of energy required to mine the next block.
First of all, the value of a BitCoin is rather more speculative than that. Secondly, when the last of block is mined, then what, the value of the energy used to verify transactions, that sounds like a cost, not a value.
How about this, Petrol coins. To create a petrol coin, you must burn one liter of petrol. Over time the cost of creation goes up, and you must burn more and more petrol to create a coin. Now you're saying the value of the current coin is based on how much petrol you need to create the next coin. I say, there must be a system that requires no burning of petrol, cause it's bad for the environment. But here's the killer, if I want to redeem my coin for petrol, I spent almost as much petrol making the coin as it's worth. So I get 1 litre of petrol instead of 2. Brilliant.