I also used to do this as a CM consultant.
I agree with keeping the cards for the customer. You know where they're at, so you can let them know when they're getting close to earning the goal- good customer service. Also, if someone lost their card, it would be difficult to say, "tough luck" to someone and drive them away as a customer.
The cards CM used to produce also had a spot to keep track of how many workshops customers attended - after so many they'd earn a free one. Good to keep them coming back and seeing new products.
I think it's a great idea for you and I wish my SU demo did it! Kim, are you there!?