I'm sure the BP also contains your BP sequence number, which is the best way of showing you "on" the aircraft.

Offering your SM number only creates a lot of IT work to be done. First, your SM account has to be accessed, then they have to search through all the PNRs attached to your account (which maybe stored on a separate platform) to find one that has the current flight in it. Then, they have to ensure you've actually checked in for the flight and find out your sequence number. Only then can they "on" you.

If you recall, when you swipe your card at the kiosk, it takes several seconds to look up your reservation and even sometimes doesn't find it. I don't think GAs would appreciate this extra lookup time for each passenger. Unless they could reprogram your card to contain your sequence number, I don't think this would save time in the boarding process. Best option actually would be to add an RFID chip to the SM cards and then store the data there. You could then just 'tap' in at the gate.