I was happy when the SEC denied the Bitcoin ETF. If you watch, every morning, as 'someone' dumps billions in gold contracts using a "market order" (ie; I don't care what price you get for my gold), you will know that the gold price manipulation is accomplished very simply and easily by 'someone' with access to unlimited freshly printed dollars.
On the other hand, we already have bitcoin derivatives on bitshares. It's a good thing that nobody with motivation to manipulate the price is using them. There is simply no volume on bitchares.org - openledger.info. If there were, then we could count on some level of manipulation.