There are so many little variables that can throw the math off, that it kind of comes down to an individual calculation for each family.
For instance, our family is a party of five adults. If I can get 40% off the rack rate price, it makes 5 nights at $350 a night rack rate, come out to a savings of about $700.00. Keep in mind I'm using these figures for simplicity, because with 5 to a room there are extra charges for the # of adults in one room.
I could pay the full rack rate and have free dining for $1750.00. In essence, the question is " can I feed MY family of 5 on $700.00.
Many people do the math and decide they would spend less on food than $700 so the room only discount is a better deal.
It depends on how much people eat, if you are really going to take full advantage of the table service meals and use every bit of food credited to your account.
The room only savings tend to be less at values and moderates, so people usually think that free dining is more worthwhile at the values and moderates and the room only is more worthwhile at deluxe resorts.
The QS dining plan at the values doesn't really save any money for many families, but some still prefer it because of the simplicity of having it prepaid as part of the package price.
The saving also go down depending on how many rooms you have to book to hold your family. If you can cram the whole crew into one deluxe room without killing each other, the room only discount may be worth it. If you have to have 2 rooms and would go deluxe ANYWAY... then the room only discount will save a ton more money.
On the other hand, if you're like me and are normally unwilling to pay the price to go deluxe... either discount can entice people to book a higher category than they would normally use, because of the savings.
So those same people may feel they have gotten a huge savings (which they did) although they may ultimately wind up paying more for their trip than they normally would.
That, of course, is part of Disney's marketing strategy.

Either way, you kind of have to do the math for your family. We rarely eat at sit down restaurants at Disney because we don't plan or book ADRS and like to be spontaneous. So if I book deluxe and get the dining plan, we would use it to eat at restaurants we've never set foot in. I may feel the money was well spent and a big savings, but I guarantee I would spend a ton more than if I stayed value and we ate on in our normal way.