I sailed on the Zuiderdam last year, first cruise. Our cruise had only one formal night, however this was a gay cruise, and that may have been the reason. We chose not to attend the formal dinner because the tuxedo rental onboard the ship could not supply the proper sized tux for my partner and we had enough luggage without the formal attire. So we ate at the buffet restaurant that night with quite a few others.
On the non-formal nights, long pants were required, however one night I realized I had never changed and was still wearing shorts halfway through the meal. Nobody said anything to me or even raised an eyebrow, but I felt self-conscious about it anyway.
On this years cruise, I think we'll find room for the formal attire, I feel I missed out and I like to take advantage of all the cruise has to offer.