Both of you are in agreement with the author of the blog, and you are all correct - if you do the math, the dining plan (especially the QS plan) just isn't worth it under most circumstances.
As far as I can tell, it's worth it only you don't have a refillable mug (in either the "classic" or Freestyle program).
If you don't have a refillable mug, it's pretty easy to calculate the costs. The plan includes one QS entree, one snack, and two fountain drinks. Each drink currently costs $3.19, and the snacks that qualify for the program cost no more than $3-4. So, if you decide that you're going to buy the 2 drinks and a snack anyway, the value of those items is $9-$10. So, that means that the entree needs to cost more than $11 for the dining plan to be a money saver. Outside of the two WWoHP restaurants, items that expensive can be hard to find.
Personally, I plan on buying the meal plan at Leaky Cauldron or Three Broomsticks, where most of the entrees I am interested in are $13 or $14, and avoiding it everywhere else.