We think of the Reward's programs as perks. Though we do generally stick with Carnival (Gold) and Princess (Platinum), we do so because we like them, not because of the Reward's.
Frankly, I'd rather do my own laundry than send it to be done along with everyone else's in hotter water than I would use, JMHO.
The past guest parties have become so crowded and the drinks are so watered down that we don't go anymore. I'd rather pay
for one drink in an hour's time than try to down 3 or 4 "freebies"
The last few times we've cruised we've been able to board in under an hour, so we really don't need priority boarding. We've never had a problem with tendering, either.
The only thing I can think of that we'd use would be priority seating on formal night, if it was offered. We had to stand in line for about 20 minutes on our last cruise. Since I use a cane and don't stand very well, I would have liked to be seated ASAP.