I've been pretty vocal about my negative opinion of NCL's new policy. But, in their defense, at least they are making it a policy. Prior to this change, they were turning people away at the door on occasion -- at least on my Norwegian Majesty cruises to Bermuda in 2005 and 2006.
IMO, if a cruise line states its policies explicitly in its marketing materials and passenger documents, I expect them to stick to it. If Princess, or any other line, is going to market itself to me (speaking hypothetically here), and I want the more formal atmosphere they're promoting, I'm going to notice if people are disrespecting the rules and that the line is allowing it. Yes, times are changing and we are becoming more casual, but that's not the issue. It comes down to expectations and enforcement.
Off my soapbox and putting my sneakers back on...