Keeping a currency's exchange rate fixed is not cheap, and considering that they're starting out with a hoard of XRP that's not worth much (for a currency), it would be hard to keep it up.
If I believe this statement, then I must also believe that the numerous people who have invested 20+ man-years of development in the Ripple system did not think through their decision on how to monetize their invention and are taking an enormous risk that the entire effort will have been for nothing.