They may be an excuse for the photographers to make money, but we do tend to get our picture taken, and do tend to buy a few. Why? Well, the alternative is to convince the entire family to dress up once a year and go into a studio for a family picture. Much easier to do it on a cruise since we're all there and already dressed (and happy too).
Since I wear jeans and tshirts to work most days, this is one of the few times a year I wear a suit or tux. I enjoy it, and the wife/kids seem to also.
If you have traditional dining and wish to wear jeans on formal night, it might be nice to ask your tablemates the day before if they care, just to preserve friendly relations.