I cruised with RCCL in September and although they only swiped the s&S card, they most definetly wanted us to hold up another ID, (drivers or passport) just so they knew we had it (they didn't look close or touch it). But they did send a lot of people back to get theirs.
My mom and I were on the Celebrity Summit, Dec 7-21 cruise and saw the same thing while waiting in Rendezvous lounge for our Excursion to leave for the Tender Boat in Cayman Island. The lady that was calling out the groups was consistently reminding people that they need to bring their Sea Pass Card.
Also on the same cruise we heard from other people that there was a person that wasn't allowed on ship in Ft. Lauderdale, FL because they failed to bring any type of ID. I don't know if that was true or not, but the above, we experienced it.