When I fill out surveys after our trips, I always comment about the need to use the information encoded on the MagicBand. I still carry a waterproof container around my neck (nerdy, I know). I keep my photo ID, AP, DVC membership card and TIW card in there. And, perhaps, a little bit of cash.
I think an earlier poster is correct; it would be fairly easy to give a MagicBand to someone else, assuming that person had his own park ticket.
Also, as the son of a retailer, I wholeheartedly agree that offering the discounts is much different from PUSHING the discounts. Many people will forget about the discounts, leaving more meat on the bone for retailers. Most souvenirs are very high markup items; Disney still makes money after giving the discounts.
That being said, Disney actually made an adjustment for me a few years ago. I went to Restaurantosaurus and forgot to mention my TIW card until after the sale was totaled. The CM said he couldn't add the discount. I kept the receipt, contacted TIW via email, and explained the situation. TIW refunded my account within a few days. All I had to do was email them a copy of the receipt.
I doubt Disney would be so helpful to retroactively apply a discount to a $250 World of Disney purchase.