There are no discounts associated with them. And if you lose them, they are not replaceable (just like cash).
But, you will get US currency and change back when you don't spend the entire amount. For example, an item costs $8.50 and you give them a $10DD. You get $1.50USD back. So, if you purchased the $DD with a rewards credit card, it's like getting rewarded for nothing on that $1.50 in change (a whole 1½¢). Plus you got rewarded for buying those Mickey Bars and other small items where they don't take credit but they do take $DD.
Lastly, they're cute and fun. The kids like them and they feel "empowered" when they can spend their own money as they choose. It makes a great lesson in saving and making wise consimer choices.