It is true you do not need the whole booklet with you at time of check in....put please do fill out the pages that they ask you to. You can rip those out, if you want and no one will yell at you.
They ask for all that info for two reasons....data collection and most importantly if there is a situation where they need to reach you before or after your cruise (cruise is late leaving, returning, etc.) I am sure it helps them to know what airline everyone is on so they can have the early flight people get off the ship first (which happened once when we were very late in getting pack to port).
There are not that many pages to fill out...
Obviously if you are not travelling with any children that are not your own you would not need to fill out the page that refers to that....
MJ