Certain dining reservations require a credit card to make. I'm also doing the Hoop Dee Doo and am also doing the Deluxe Dining Plan. If you are a no show at your ADR for this reservation (or whichever one requires the card), you will be charged a fee of $25 per person for the cancellation/no-show. I know for a fact that as long as you phone in your cancellation, they won't charge you even if you're within the 24 hour period. One summer we had ADRs for California Grill and that also required the card to make the ressie. We called and cancelled the same day that we were supposed to eat there and it wasn't a problem.
I believe WDW requires this now for some of their more popular restaurants that are signature dining experiences. I wish they would have a credit card required for making reservations at Le Cellier, too.
I cancel all ADRs that I know I'm not going to make, whether they require a card or not. I believe it's just common courtesy to allow somebody else the opportunity to dine.