In some hotels you need a card to swipe for the service. But that can be detrimental in a fire. I have used the service elevator many times in hotels and conference centers.
I have too. In his position, I would not have used it for bringing supplies up to the room, but it could easily have been my primary escape route. Even if it looks clear, the public elevators and stairwells are almost certain to be monitored, while the freight elevator has a (small) chance to be overlooked. Or perhaps a service elevator to the second floor (if that is public areas rather than rooms), and then an open staircase instead of an emergency staircase that goes to the top for descending that last floor.
I have, but only in a convention
Do they use elevators in a fire? That is explicitly forbidden where I stay, but most of the hotels are not high rises.