When Cruising started up again, there was an effort to keep servers from touching peoples cards. They might glance at the card, or they might ask for your room number. Recently, I have seen some servers actually take the card over to the point-of-sale machine, some have just asked for room numbers, and others have asked to see the card, so anything is possible.
While the app works for many things, it doesn’t replace your card. It won’t work for getting on and off the ship at a port, for example. And fairly often I experience times when the app simply went down and was inaccessible. You would need your card to get in your room if that happened to you.