The dress code matters a 10, to me. If I wanted to dine with people who dress as if they were eating in their back yard I would not go to the dining room on a HAL ship. Bravo to Ron, I agree with everything he said. Once a dress code is abandoned you can expect anything. Just look at the schools.
If putting forth the effort of dressing nicely(even on casual nights) for the dining room is to much, then go to the Lido. I believe you accept ALL the rules of the cruise line when you book your trip. If you don't accept these, then go else where. When you break the rules and forget your manners you infringe on other people's enjoyment.