It would be interesting, if somehow the exchange functionality were built into the protocol and entirely P2P. Getting rid of these centralized exchanges seems like it would really stabilize the currency. I don't know how remotely feasible that would be, it is far too early in the morning without coffee.
Effectively, what you describe is the protocol(which is p2p as well), except that the protocol does not address the issue of matching prospective buyers and prospective sellers (a protocol that does this, without a central market-operator, would be neat; but bodging one into a protocol for transferring virtual coins without double-spending would probably be unwieldy, though clients that implement both would be logical).
Uhm... isn't Ripple supposed to be that decentralized exchange?