Another consideration I believe whether they even allow to be sold single access on-board is number of length of cruise passes they have sold (which are only available to be bought on-board). Length of cruise passes make sense on a 6-7 day cruise if you see yourself using virtually every day. If not, the individual day passes pre-purchased for the number of days thought you will use makes more sense.
We've done length of cruise on a 7-day, individual on a 3 day (we did not even use up) and last time bought the length of cruise pass only to have refunded several days into the cruise (not having used yet) when it came down to deciding between a booked spa service and using the length of cruise pass (they were good that way in refunding - sometimes there is just too much to do and decide between on-board and there is never enough time to do all). Talking the Dream class ships, we enjoy the rain-forest showers, sauna's and warming beds but the hot tubs have always been hit or miss if working, being hot enough or enjoyable depending on the ambient temperature.