The way I understand it, Ripple is just a centralized/decentralized hybrid exchange. People still have to valuate their currencies, and put up ask and bid prices for it to work. And there will still be costs involved (from the spread in exchange rates, fees from the gateways, and risks involved in trusting IOUs, the owners of Ripple, the merchant you are paying, and the government jurisdictions all these groups operate in). I suspect that holding IOUs for a long amount of time would be very risky (the issuers may go out of business or be shutdown).