Then you don't need to deal with problems like a seller being matched up with 30 partial orders and needing to figure out how to pay them all through fiat infrastructure.
I don't get your logic. Another currency does not solve the centralized inferstructure problem.
The decentralized exchange will allow for p2p exchange of any currency for any other crypto or not... Gold, silver, and any other commodity of value too.
....Solves the exchange problem for all the existing crypto currencies as well.
DarkCoins would be easily divisible, and able to be transferred quickly/easily, much unlike 12 bags of gold flakes. Fiat transfers, even with a decentralized exchange, have banks as a point of failure. If you're trading with people directly, as I'm assuming would happen in an exchange, the "decentralization" is proven failed the instant BAC freezes your account over AML concerns with 20 out-of-country people deposited a combined $50k into your account over a couple days. Gox, OTOH, is a central figure with a relationship (one they've incidentally had to fight hard for) with their banks, which no longer freeze their accounts every other week.
How about this http://en.wikipedia.org/wiki/Smart_contract ?