In my opinion, Formal nights are an excuse for the photographer to make some money. The "picture" locations are set up throughout the ship and are very difficult to ignore. They assume that since you are "dressed to the nines" it would be a great opportunity for you to have a photo taken (in excess of $20).
My husband and I usually dress for dinner, then go back to our rooms to change clothes before heading for the casino in our jeans.
I absolutely HATE to pack formal clothes because it takes up too much room in the suitcase. I have to dress everyday for work, why would I want to dress for my vacation?
There are many positive and negative comments about formal nights. Do what you feel is best for you.