I opted to pay the extra and get the card in addition as well for the very reason to have it while ashore. Not like I'm going to be driving to Mexico or Canada. I remember asking about this as an additional ID and Dave said it was a good idea. Bravo!
If nothing else it would expedite problems should I ever miss the ship on foreign soil or something unfortunate would happen. Passport? Mine is locked in the cabin safe on that ship that is sailing away. Here is my card though if that helps.