I don't want to decide where, when and what I'll feel like eating six months from now. My kids don't tend to eat on a regular schedule so if they want lunch now, but our ADR isn't for an hour, that won't work for us. Same thing if it's time for our ADR but they aren't hungry yet. And if we want pizza but I've booked seafood?
We don't want to have our park time tied to an ADR...if we wake up and want to go to MK but our ADR is in Epcot, that would annoy me (we don't typically do park hoppers, and even if we did it's a pain to worry about travel time). Plus the kids don't want to sit still through a TS meal when it means taking time away from park activities.
Too much food. We don't normally eat an app., entree, and dessert. Even two of those three is a lot. Nobody drinks soda - it's all milk and water, with the occasional lemonade for DH. We'd end up either overeating because it was there, or wasting a lot.
And most importantly, the dining plan costs me more money. We bring a lot of food (DD #1, especially, has a bit of a nervous stomach when we travel...I've learned that we're all a lot happier when she's able to eat familiar foods. With what I do end up buying in the parks, I come out way ahead paying OOP even compared to the QSDP.