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.
But they don't have to keep the exchange rate fixed to some given currency, they will profit anyway as long as enough people start using Ripple. If the transaction costs are a major hurdle in getting people to start using Ripple then can push down the cost / spread out more XRP, but unless the major use of Ripple is to hold XRP as a store of value or make transactions in XRP, keeping the XRP rate fixed relative to some other currency shouldn't be all that important. Users won't want the transaction cost to fluctuate orders of magnitude overnight, but that goes for holders of any currency.