The instructions do say to not use the sail and sign card and the issue is security, but the room number is not on the sail and sign card. There is a 4 digit card number.
There is still risk; when you order a drink and they bring you your receipt, your name, card number and room number are on the receipt. So in order to get into your room and into your safe, somebody has to order something and charge it to your card 1st.
Within a day, you'd be surprised how many of the crew members know your name. These people work around the clock and they are all over the ship. They do look at who is using these cards everytime you charge something.
But for best security, use your AAA card or drivers license. It's pointless to carry around a credit card since you can't charge anything with it.
What is also risky, is leaving receipts laying around. Many of the bartenders and waiters will charge drinks to your room if you simply give them the card and room number. But they will require both and the receipts have that information.