I think the DDP is worth it, for us. Every family has different needs so assess it according to your eating styles. We paid for the dining plan last year and I think it waay more than paid for itself. In fact, on the last day, we still had over 10 snack credits left so we had lots of frozen lemonades:slurp: and took a few snacks to the airport. Not having an appetizer is a bummer(personally I say leave the dessert and keep the appetizer but I get that a dessert is cheaper than an appetizer), but I still consider it a value. I was looking at the check of every meal and we really did save. Plus, as others have mentioned, I like not having to worry about a food budget. It's already paid for. Sure you can prepay on your KTTW card and link it to a credit card, but even then you can use that $ for anything in the world. I think it is a no hastle way of having everything done for you. (Like having vouchers fro everything with no vouchers to hand out)
As far as exchanging a TS for more QS...I don't think you can do that but call 1407-WDW-DINE to varify.