Actually I don't think $50 with the tip and appetizer added back in would end up being that great of a savings (or at least one would have to look carefully at the kinds of TS meals one has and exactly how expensive one's choices are likely to be). The PP's calculation lists $40 for a TS entree, but aren't there very few TS restaurants that have entrees near that cost--I know Le Cellier is an expensive one--(actually even at the 2TS restaurants we've been to we've never paid that much for an entree), so even if one is ordering the most expensive thing on the menu, I think the savings will still be significantly lower.
And if people aren't actively trying to order expensive things/go to expensive restaurants/order every course at every meal, then there is even less savings. Consider a fixed price meal at Boma or Ohana--26.99. Keeping the CS at 13, and the S at 4, and adding in tax and tip that comes to $50.87. At Whispering Canyon if one gets the meatloaf and soup or salad for an appetizer the cost of that meal would be $36, or $53.50 for the day.
Personally we'd also figure in to the calculation that we aren't always going to maximize the CS and S credits. Often the desserts at CS are a total waste for us (we've asked a couple times to substitute something else like coleslaw or fries, but were told fruit was the only option)...we'd rather just eat a sweet snack which often tastes a lot better than the desserts offered at a lot of the CS. And usually we each get a drink and then end up tossing one of them, since we really can only drink one between the two of us. So that brings the CS value down about $4 for us some of the time. And sometimes the snack we want is only a $2 or $3 item.
On a day like that--cheaper snack, wasted drink and dessert at CS, and less fixed price dinner, we'd only be getting something like a $48 value out of the
DDP for the day. Our days would probably end up mixed--some cheap days, some days when we maximize the credits. So in the end we could probably save $50 or $60 for the course of the vacation for the the two of us using the plan. Not actually that great of a savings given one has to pay upfront (taking a chance, for instance, that one's family might get a stomach virus and end up wasting those credits). For us it's definitely not enough to make it worth it to give up other discounts and be required to buy at least a 1 day ticket.
And really, the two of us could eat pretty well on our own $100 a day budget and probably could actually get in a few extra TS meals (sharing or skipping) appetizers and desserts above what the DDP would give one on that budget.