These days I would have to say I "tolerate" formal nights. If I am cruising on a line that has a true "Formal" night I will dress accordingly.
If it's Country Club casual, such as Azamara, Oceania, or NCL then no jacket goes.
If it's a "Cruise Elegant" night on Carnival I probably won't wear a jacket and tie. On my upcoming Carnival Breeze cruise, and subsequent European vacation, the jacket and tie will stay at home. I will be nicely dressed on Elegant Night but without a jacket and stay well within their dress guidelines.