Unlike bitcoin[which has no use value, only trade value] xrp is needed to use the functionality of the ripple network.
Well by the same token Bitcoin has also got use value because it speeds up transactions if used as fee and is also needed for some transactions to ever go through.
That's an interesting interpretation however that's still a trade value -- the bitcoin is merely changing hands from node to miner, so realistically that's still an optional trade, and hence still kind of a trade value. That's splitting hairs though so I'll admit you have a point.
The fact that BTC fees don't dissapear doesn't really matter. XRP's dissapear and their value simple get's spread over the remaining one's, everyone get's richer at the same time (well, richer in IOUs that is). The difference is that in bitcoin only Miners get richer since btc fees don't dissapear but are instead collected by them. ;-)
Also don't forget that when Ripple gets open sourced (I hope it does in the end), everyone will be free to reimplement the protocol so that it doesn't need XRP to function. And I think this should definitelly be done to properly decentralize the network.