The justification depends upon your dining habits. We seldom order appetizer, entree AND dessert...it is too much food for us for one sitting. We also often have a TS breakfast at Olivia's, and a late TS lunch in the parks...the dining plan would not cover both meals.
Based on what we ordered, including tip, for a 9 night stay we came out $5 cheaper by using the DDE than we would have using the dining plan. That is why I encourage folks to add up there meal costs for a typical trip and do the math. The best way to do this is simply add all the dining places up from your room charge bills.
Plus during the trip, we ordered 5 glasses of wine, which I included in my total for our food...they would be out-of-pocket in addition to the dining plan. So, if I didn't count the wine, and based solely on our dining habits, the plan would have cost us about $30 more than using DDE. The plan is of no value to me if I feel like I have to over eat and feel "Thanksgiving stuffed" every day, and I'm a rather rotund guy, 5'8" 250 lbs. We also, as adults, don't do character meals, which are pricey to cover the cost of the CMs in character. There were some days we spent more $$ than the dining plan would cost, and some days we spent less $$, overall, it would not save us anything.