MtGox is essentially doing what you are talking about, allowing people to trade bitcoin derivitives (that they supposedly back 1:1 with real bitcoins). Trades on MtGox don't hit the blockchain.
A great plan if you want to buy coins for $632.58 and end up getting $260 for them because of a centralized system failure in an inherently decentralized protocol.
There's an old saying about idiots, idiot proofing, and long term trends in idiot breeding under idiot proofing environmental conditions. So invent a decentralized massively redundant system, someone WILL find a way to centralize it, and SPOF it, yet still blame the designer when it blows up. Like a law of human psychological nature.