Why would you pay a trading fee for Ripples? Some exchanges (including BitStamp, JustCoin, Kraken, and many others) charge a nominal 0.5% trading fee for matched orders. However, it is also possible to trade directly on the Ripple network with the same order book by using the Ripple client and setting the issuer to the respective exchange, thus avoiding the fee. Why would anyone pay a trading fee? (Other fees, like deposit/withdrawal transaction fees, make more sense to me.)
For the same reason you'd choose one exchange over another (in general) even if the other has lower fees. Examples of reasons not related to trading fees for which one might prefer a particular exchange:


Higher volume/liquidity
Better ergonomics
Faster trade execution
Easier deposits/withdrawals (especially when fiat is concerned)
More trading pairs
More reliable