Weighing in, and likely repeating a few points, but...
Note that "
free dining" is a package discount, just like any other discount available. It's just a little more peculiar. From a financial standpoint, free dining tends to be a better deal as the grade of accommodation goes down and the number of guests, particularly adult guests, per unit goes up.
As noted upthread, a family of 4 staying in a value or moderate resort is likely going to see a greater package savings with free dining than a room-only discount, while 2 adults in a deluxe resort will make out better with a room discount.
Of course, the savings presumes you want the dining plan in the first place. The subjective value that you might assign to having the dining plan (especially "free dining") may not be the same as the list price of the plan. In that case, you may see your perceived savings benefit lower than your actual savings.
Furthermore, if you were originally planning on not having a dining plan at all, you will actually spend
more money on your reservation than you otherwise would have, if you could apply any discount off of rack rate at all. Of course, then you have significantly less out-of-pocket costs for food and beverages while you're there.
So, to the original question, "is free dining worth it?", the answer is "it depends", for all of the reasons noted above.