The DDE is a discount program that affords members 20% off of their bills at many (but not all) WDW restuarants. The DDP is a pre-paid meal program that gives you "points" to spend for meals to completely cover the bill. You cannot combine the two.
We had annual passes in 2005 and so used the DDE to save some money since we were not elligable for the free DDP that was being offered to hotel guests at the time. It was a better deal for us to get the AP discount on our hotel and get DDE.