At online checkin, try choosing one bag, and see if a fee comes up (it shouldn't if the MP# on the reservation matches your MP# linked to the card)
Unofficially now, I added my father as an additional cardholder to my account four years ago. While the free bag benefit should not apply in his case, he has always gotten it waived at the airport.
He travels twice a year, and has been able to always show the card, and get a bag for free. And this included checking in with United and Continental agents, as well as a skycap.
Even better is the fact that I have closed that account six months ago, and he still gets the free bag.