The WDW restaurants are usually very accommodating of special dietary concerns, just let them know if you have party members that are diabetic, have allergies, gluten intolerance, etc, and they will do what they can for you. I've heard you can even call ahead of time to have it noted on your ADR.
As for the length of time it takes for appetizer and dessert courses, that may very well be one of the main reasons for no substitutions, but it's actually a bit lame if you think about it.
If my husband and each order an appetizer, then it might take 15-20 minutes for that course; same for dessert if we each have our own dessert. But if we shared an appetizer, it might take us 8-10 minutes for the course, since we are each only eating half of it -- and the same for shared dessert. So what's the difference?
A few minutes extra for taking the order, prep time in the kitchen, and bringing it to the table. But there are variables for any party eating in a restaurant -- some people will eat in a hurry and leave, others will want to linger, chat over coffee and dessert...
I personally think one of the biggest factors as to why the
DDP is dessert only with no substitution for dessert is that guests are more likely to pay OOP for an appetizer at the beginning of a meal when they first arrive at the restaurant, they are hungry, everything looks good. They are more likely to go outside of the prepaid DDP and order an appetizer that they will have to pay extra for. If they were given a choice, they might order the included appetizer, the entree, the beverage...and by then, they will be full, and satisfied, and a lot less likely to open their wallet and pay for a dessert course. Also, if you think about it...if they order an appetizer OOP, eat their main meal, and then they are too full and don't bother to order the dessert that is included with the DDP, then Disney saves money, right?