I think I'm taking this a bit off topic but what exactly is then the point of having a "de-centralized" exchange ? Either Ripple scales well for arbitrary trusted gateways in which case it does add a valid service or all you are doing is running additional ripple software on existing exchanges at the benefit of those who hold XRP.
If you rely only on a hand full of gateways ripple adds virtually no advantage, on the contrary you now have to trust both the gateways and the software itself to work as expected
Hundreds if not thousands of gateways, to my understanding, is the goal.
One of the advantages of the distributed exchange, is to allow payments from any currency, to any currency. The network will find the best path by utilizing this distributed exchange.
Gateways are also advantageous over exchanges, because their Ripple transactions are all listed in the public ledger. So at anytime, you can see how many IOU's are outstanding and how much cash reserves the gateway should have. Good luck getting that from an exchange or bank.
XRP is advantageous because it can be stored with no counter-party risk and can move through the network, without trust lines. It can also move into or out of any currency on the network and is known as a bridge currency.
Let's say you're going to send me payment, which I require in EUR but you want to spend BTC. It's tough to convert BTC - > EUR and then send it to me, without Ripple. Takes a long time and it's expensive. With Ripple you can for instance:
You -> BTC -> XRP -> EUR -> Me
That would be one possible path through the distributed exchange. And it all happens in seconds, for a fraction of a penny.
If you're a person, who in the future only plans to deal in Bitcoin transactions, Ripple is of no use to you. If you're a person that plans to deal in multiple fiat and crypto-currencies, Ripple is an amazing payment network.
More info here: http://en.wikipedia.org/wiki/Ripple_(monetary_system)