Besides opening your door, what exactly do they do on the ship?
That's mostly what we use ours for. You can also use it to charge things to your room, as others have mentioned. They will light up and "interact" with some shows/restaurants, if you have them charged up.
On our last trip, I never charged it, I just left it on the whole week. I like that I don't have to worry about where the card is when we're in the pool, etc. Mostly, I loved not having to remember on my way out the door if I grabbed it or my card because I just had it on at all times. This last trip was also super windy, so I would have gotten annoyed with the lanyard quickly. I don't always have pockets, so I don't like having to carry the card if I don't have to. It will work for everything except the lighting up without being charged.
As some mentioned, in most ports now, you can use it to scan back in at the ports. We end up putting our cards in the safe with our ID's so that we grab them when we're leaving the ship, just in case.
All that being said, I don't think they're necessarily worth spending money on. We got ours as gifts from my in-laws. We had them on our list because we knew we had a bunch of cruises coming up. We've now done 4 cruises with them and I prefer it. However, I don't know that I'd recommend it for just one sailing or if you had a large group and had to spend a lot of money on them.