preferred customer card

[partz]

I am looking for your thoughts-pro and con about a "preferred customer" card to be given to customers at time of new or used car purchase. It would offer a 10-15% discount on parts and service and maybe some other things.(oil change, detail etc.)
Are dealers that are using something like this seeing an increase in sales or just giving profit away? Any problems with RO's or invoices at the cashier?. Thanks