Actually, we do. The issuers in such a system take bitcoins or any other asset they're willing to hold from you, store them and issue tokens which then can be traded (just like the IOUs in Ripple).
When you're done with your trades, you can take the tokens and redeem them for the real thing at the issuer. So as long as the token circulates within the exchange, the issuer stores the actual value (we hope). So far this is all not very surprising if you know how Ripple works The main difference might be that this approach aims to achieve a truly decentralized exchange network, something that Ripple didn't fully achieve yet as far as I can determine. However, I don't know whether it is possible to build a blockchain-like thing that performs well enough for block times in the area of 10-60 seconds.

Note that my Ripple comparisons are not meant to endorse that system - it's just the distributed exchange system that I've studied most, so I'm using it to draw analogies and try to understand how things are supposed to work.

Onkel Paul