"Backing" bitcoins or any currency with a single resource is never a very good idea since any single comodity can be controled. if a single wealty enough individual wanted to they could inflate and deflate the vaule of bitcions at will. look at our market cap it's less than a hundred million dollars one or a group of sufficently rich people could control the entire market fincially. If bitcions were to be backed by any single comodity the same thing could happen right now however they are treated as a comodity themselves not so much as a currency.