As I see it even the most strict of dress codes have become a thing of merely being a suggestion lately so why bother lugging along all the extra clothing when a casual attire will still get me seated in the DR.
A cruise ship can hardly be compared to an invitation to someone's wedding, being in the military or dressing as you employer wants you to. It's simply a vacation where you've paid your fare and as long as a person is neat and clean there should be no reason to force someone to comply. On our last cruise there were quite a number of people who chose not to fully comply (some not even close to casual on a formal night) and were welcomed in the DR with open arms. It's strictly a business and unless you're dressed in shorts & a T shirt on a formal evening most head waiters will turn a blind eye