Well, I guess I'm not exactly comparing apples to apples - for all the food you get on the DDP, the cost may not be that different. But on our trip last week there were 5 of us (3 adults, DD-12, who by Disney standards is an adult, and DS-9). Had we purchased the DDP for our 6-night stay, it would have cost $977.70. Using our DDE, we only spent about $750 on meals. Granted, we did not order appetizers and desserts at all our TS meals (tho we did occasionally indulge in dessert), and other than snacks it worked out to 7 TS meals each (8 if you consider our dinner at Cindy's would have counted for 2 TS credits) and only 3 CS meals (we ate breakfasts and a lunch or 2 in our villa) but we all had plenty of food. So FOR US, the DDE was a better deal than the DDP would have been. We would have wasted food using the DDP. That may not be the case for everyone.
HOWEVER, if I got a 20% DDE discount on the DDP, it would have only cost us $782.16. THAT would have been worth it (tho we still would have had food leftover because none of us would have eaten that much).
I know Disney will not offer this - I just think it would be cool if they did.