But really... Is Ripple going to come down to the user trusting a few servers? One of the primary strengths of bitcoin is that the blockchain encourages competition among nodes and provides an incentive for all participants. Where is the incentive with Ripple?
There is no incentive in Bitcoin to run a client. The only incentive is for mining, which doesn't exist in Ripple.
Will it be left up to the big players in Ripple to provide servers in order to keep the value of the currency going? In that case, how is it decentralized? If there is no mining of blocks, then it sounds like ripple would be unable to provide transaction fees to those running servers... where is the incentive? Nobody is competing for blocks!
People running clients don't get any compensation in Bitcoin. The Ripple server is analogous to the Bitcoin client.
People will run Ripple servers because they want access to the Ripple network. Otherwise, they'll have to find someone else's server. People who run servers to provide access to the public will expect to provide access to hundreds of low-load clients, not a few high-load gateways, arbitragers, or market makers. The only way to ensure a reliable high-speed path is to run your own server (or pay someone else to do so for you). Then, people won't cut you off because you'll be pulling your weight and cutting you off would mean cutting themselves off.
Once you're running a server that tracks the network, the extra work to validate is negligible. It's just a few crypto operations to sign proposals and validations.
But I do agree that there is a real risk that the network could shrink to a small set of validators. I don't think anyone has any incentive to see that happen and I think everyone who uses the network has an incentive to see that that doesn't happen. We'd like to see a large number of validators around the world run by different groups with different interests, just as we now have a large number of Bitcoin clients running around the world run by different groups with different interests.