For the cruise... your TA and CCL reps. you have spoken to are correct... problem lies IF you miss the ship and need to catch up with it or have to fly back home from a foreign port of call... thats when the passport would be required.
Bite the bullet???... thats a financial issue only you know how to handle. Perhaps to purchase say the adult passports first then add another family member passport purchase later in the year.
Ther will be many that swear you must have that passport, but in the end, only you will know what you are willing to risk or expense. Good Luck