1) Conveince. I like that everything is prepaid except tips. Its easier to pay tips during vacation than the entire meal.
2) Flexibility. We go DDxP now. This allows DH and DD their large breakfasts every morning and myself a TS every evening. DH likes the apps, I like the desserts, DD9 likes it all!
3) Enjoyabilty factor. If we did not use the plan, and paid as we went, I'd end up choosing cheap resturants, or the cheapest entree. By nature I can be a cheapskate, especaily when traveling because I worry about the "just in case" factor. By using the plan, everyone gets what they really want, and I forget about the prices; because well, it's already paid for.
4) Savings. This summer if we ate OOP the way we ate on the plan, we would have spent $400+ more on food than we actually did. Now if I had not been on the plan would we have eaten the way we did? No, because DH would have skipped his apps and I would have skipped my desserts. DD would have been begging for dessert every time and NOT gotten it, to save money. How did I know it was worth it before I left? I printed out the menus for the places we wanted to eat at, and did a guess of what I thought we''d likely order and ran the numbers. This showed me on entrees and character buffets alone I saved money. When we got home I ran actual prices, and saw my savings.
5) Better food choices. By using the plan, we ate in better resturants, thus giving DD9 better choices than mac and cheese, nuggets, or meaty pasta. While she loves "kids food", I don't let her eat that way as often as she would during a week vacation @ WDW. However, when presented with better choices, she usually will choose fish, grilled chicken, steak and even sushi! (I guess she's does that because wherever/whatever we eat, she eats! I don't fix seperate meals for her at home, or take a mickey d's into what my friends call "adult" resturants.)
6) Memories. By using the plan, we tried new resturants. This gave us new experiences, and new favorites. We didn't stick to the tried and true because if we didn't like it, it was okay we had extra snack credits for later on when hunger hit. (BTW we liked every where we tried this summer. Some better than others, but really no bad meal).
IMHO the DDxP is the way to go if it fits your budget.

Personally I'd never do the quick service plan, because that is my least favorite type of meal.

We've done the reg. DDP prior to this summer (in Jan was the last time) and I ended up frustrated trying to get DH a good breakfast, while preserving the TS for a good resturant. The counter services got old by the 3rd day. Even DH, who likes sandwhich type meals said enough of that by the time we went home. In addition, we spent more money OOP on the regular plan.