On one of our cruises we went up to the Horizon Court in our casual attire and had a nice formal dinner night there. That was our choice.
We dressed for the second one, as we were going to be eating with friends in the Dining Room. That was fine and we took pictures of each other.
We stayed in formal attire all evening on one cruise but by the end of the night, we were quite uncomfortable.
So, on average, we tend to change after the formal dinner, as we don't get the professional pictures taken and usually end up in the Casino.
Last year, one of the couples at our table "forgot" it was formal night but came to the table anyway. He was wearing a collared shirt and she was dressed all right. It didn't detract from our group's evening.
I just saw pictures of my friend's recent cruise. She and the other gal were dressed up for formal night. The men were in shirts and ties, no jackets. I don't even think my friend's brother was wearing a tie!
My final answer--keep formal nights formal, for early cocktail parties and dinner at least. After that, I think the choice can be made individually concerning staying formally dressed. Casual should be allowed all other times, but shirts should be collared. Clean, good quality jeans should be allowed-with a belt. Shoes should be required-no tank tops or shorts. In other words, it would be nice if we were all neat and clean for dinner.