I believe people probably have been turned away from the dining room on formal night who were totally flaunting the rules in shorts/jeans/t-shirts or the like. However, I doubt very much that anyone has ever been turned away who was dressed nicely (dressy casual, business casual, whatever you want to call it) just not formally. Celebrity cares too much about their bottom line to ever start alienating paying customers in this way.
I fully understand that Celebrity has the right to turn me away if I am not dressed up to their standards of formal and I would accept it gracefully if they ever did. However, they never have and I doubt they ever will. Their lack of interest in enforcing their "guidelines" suggests to me that lesser styles of dress are acceptable to them. The bottom line is that my agreement is with Celebrity and how I choose to dress is between me and them. I don't feel the need to dress in a way that pleases total strangers.