Actually, for reservations that are generally pre-paid (like CRT), it can help. As I understand it, if you DON'T tell them, you've already paid for the meal, so they return the discount to you in a gift card. If you DO tell them you have the DDE-er, TIW card, they mark the reservation as if you have
DDP - you don't pre-pay, but they do keep the credit card on file in case you don't show. You then pay at the table. That's what they did for us...but it even confused me when they asked us for our KTTW cards for the DDP and I had to explain...