They can also split up families and have you on two different flights if the need arises. Book your own and fly in the day before. I believe this to be a better option.
I would be nervous about flying in the day of the cruise if I booked my own flights. In case of delays we are out of luck. That is why we always fly in the day before and relax at the Radisson at the Port and they board the ship the next day.
Save lots of money and stress.
MJ