Every time I crunch the numbers I come out slightly ahead with the DDP, but slightly may not be enough to make the disregard the convenience of not thinking about who has to order what, just getting what we want and paying at the end.
That's why we decided not to do the DDP again.
We did it once when travelling with another family that included a very "budget-minded" dad.

His DW knew that there was no way he'd pay the price for character meals, etc. if it was OOP. So, the pay-up-front style of the dining plan was an advantage on that trip, and enabled his DD to go to all the character meals since "they were already paid for, anyway." (Can you tell us wives did all the planning?)
But it was a real pain, IMO, figuring out what could be ordered and what couldn't, whose cards to take how many credits from, how to use up all the snack credits, etc.
With the room charge, there's no hassles...just order what you want, and charge it to the card at the end of the meal. No figuring out who can order what and feeling obligated to get desserts at every meal even if you don't want them, no rush to use up your credits at the end of the trip.
We did the math, and the small amount that we'd maybe save isn't worth the aggravation of using the plan in our books. Even during
free dining, we usually go with the room discount instead...it's just better for us overall.