The "savings" amount is always a tricky proposition. When we went last year on Deluxe (2 adults, 1 child, 2 infants), we probably saved 20-25% over what the same food would have cost us without the plan.
But we almost certainly wouldn't have ordered the same things, or done the same restaurants, if we were paying for each individually OOP.
On the whole, actually, we probably paid significantly more to eat for that week than we would have if we had been doing the whole vacation OOP. But the extra cost came with a change in the vacation experience -- we were able to try a lot of places, get the kids the character meals, order things we might not have otherwise tried, and do a lot more table service over counter service. We also liked the convenience of having it prepaid so it was a simple matter of using up what we already paid for.
Now, that's not for everyone, and someone who places a higher value of getting the most vacation for the least money would be better served not getting a dining plan at all (or even staying on-site, truth be told). It's a matter of budget and priorities.