It all depends. If you have 4 people in value resort, FD infact works better, but for 2 people it would be different, for higher category it would be different, for deluxes it does not make any sence at all.I have to think you must never have considered the actual math.
We had free dining for our trip in late 2010.
We stayed at Pop, so our room for the seven nights (before taxes) cost $685. With the 25% offered discount, we would have saved $171.25.
The quick service free dining for two adults and two children was worth a total for seven days/nights of $587.72. (For the math challenged, that saved us more than $400 more than a room rate discount would have. Even if you assume you would never spend quite as much on food without the plan, it saved us significantly more than a room discount would have.)
On the dining plan, we were able to eat every single meal, have every single drink and snack, and still have credits left over for a bunch of extra "free" fudge and other treats to bring home. Unlike a room discount, it also added an element of fun to our vacation, because my kids knew they could each pick two special treats a day. They enjoyed figuring out which fun stuff they wanted to try.
I think those who poo poo the Disney dining plan must stay at deluxe resorts. Those are the only resorts where a room discount might be a better deal than the dining plan. Even then, it doesn't add anything to the vacation.
It also depends on how you eat, we have 3 adults and we do not eat that much food, infact we share and not because we try to safe but because it is just too much for us. We also do not spend money on soda and do not eat that many snacks. I do not want pay extra to bring snacks home, I prefer to keep my money in a first place.
Another down side of DP, free or not, I do not like to be locked in selection, I do not care for deserts but love to have appetizers.
We also do not like that we would have to make ADRs for each and every TS meal, we still eat TS meals but when we feel like it and we always have alternatives if place is busy.
Also do not forget to include tips in your calculations, it adds up dramatically and something people do not always consider when calculating discounts.
So, it is not that obvious which discount is better.