We were just on the Jubilee and it did require getting a key from our room steward in order to use the small safe.
There are times when your room steward is cleaning your cabin where he/she might have to run down the hall for something. It's those times that concern me. When our door is open to anyone - if they are quick enough. So our most valuable items do get put into the safe. We've had safes that have had key pads to set your own code, key locks, and magnetic strip locks. I don't know that I'd use my sail and sign card for my safe....what if you lost your sail and sign card...although it's difficult to tell your cabin number. These are just precautions. We've never had any reason to not trust our cabin steward....after all if anything comes up missing they will be the ones to get the blame.
Most are honest....the precautions are...just in case.
By the way...on the key safes....there seems to be only one key...and you are warned that if you lose it you will be paying $40 for it's replacement.