I agree that the Dining Plan isn't really about saving money. It is about convenience, but it's also about another thing that kind of goes along with that.
When you're sitting at home planning on your vacation and your budget, you might tell yourself, when you don't have the dining plan, that you'll drop by the convenience store for breakfast/snacks, hit a QS restaurant in such and such park for lunch, maybe hit a fast food place or chain outside of the parks for dinner.
That plan will undoubtedly save you lots of $$$ over the dining plan and if money's tight, I'd recommend that strategy and you could even go cheaper.
But, when I'm in the parks and everyone's tired and getting cranky and hungry and there hasn't been an official time set up for a meal (like through an ADR), the money-saving plan quickly goes south.
Then there's also the issue of some QS being unexpectedly closed when you're there or lines around the block, etc. Although, that can certainly happen on the plan as well.
The other issue is that when you go cheap, you often wind up eating a lot of the same type of food and it's often not the healthiest of choices. Your mileage may vary, but I think the Dining Plan keeps all the meals interesting.
And I love planning our eating strategy before we go just as much as I like planning the other parts of the trip.