Since virtually all Disney owned sit down and buffet restaurants participate in DDE, as well as counter service at AK and resorts that do not have a full service restaurants, there would be very little for them to "add".
The original purpose of DDE was not as a discount card for every WDW guest that has an AP, but as an incentive to get local Floridians to consider having a nice dinner at a WDW restaurant...since prices on property are higher tha off-site and it is a bit of a drive from Orlando.