For the last few months I have been working on an economic/business project based around Disney dining prices & the Dining Plan.
When all the data is analysed in 2011 the average family (2 adults/2 kids) did save money based around the 2011 prices although best estimates were only putting the saving at around 7-10% certainly not the 25% advertised. However, it WAS possible to save around 25% if everything is maxed out. The interesting concept was that all the savings came from the kids plans, not the adults. Really as an adult in 2011 you were coming off even & it was simply a way to pre-pay for food rather than otherwise. As others have said it was purely convenience to the average guest.
In 2012 things just go out of whack. I haven't spent too much time looking at data for 2012 but early indications are that really those using the dining plan are losing money. Although I haven't worked on all possible permutations.
As a real world example, using my actual plans for my family's 2012 trip (4 adults, 21 nights) we are estimated to be at a
$800 loss. Then to put on top an approximate $350 in tips not covered in the plan. Fortunately for us we get
free dining.
At 2012 prices it's hard to see how an adult even comes out even & children look like probably coming out about even. 2012 sees the year when OOP is the better value.