We usually do one formal night on Lido Deck dressed casual (jeans, dockers, etc.) and the other (lobster night) in the main dining room dressed in what I call "dressy casual" (plain black slacks and a pretty blouse for me and dress slacks with either a shirt/tie or a collared shirt and pullover sweater for my husband).
With each cruise I notice the casual alternatives getting more and more crowded and the main dining room dress getting less and less formal. It seems that, in one way or the other, many people are opting out of dressing up. I predict that someday in the not too distant future it will be the formal diners looking for "alternatives" while the casual diners enjoy each night in the main dining room. I sure hope so!