On the other hand, I think the excuses those who want to insist on formal wear are BS
1. It shows respect- This is not a wedding or event you were invited to where you respect the host. The cruise line is not your host, they are a paid vendor.
2. Formal night/dinner deserves formal wear - Stop kidding yourself that this is a luxury experience. You are not in first class on the Titanic. This is a mass market cruise and nether the ship, meal, or entertainment is even remotely forma or luxurious.
3. It's our only chance to dress up - Where do you live, in a cave? Go out to a formal event or high end restaurant, opera, etc. and dress-up.
4. Those who don't dress formal, ruin the evening's atmosphere - Actually, much worse than seeing someone dressed casually, is men in tacky polyester rented tuxes with plastic shoes, men squeezed into tuxes they have had for years which are 3 sizes too small with gut jutting and hanging over their waist band, women who think polyester black pant/skirt with a tacky bedazzled "top" is formal.