I'm one who does have a problem with jeans on casual night -- but only IF the cruise line states it doesn't allow them but pax wear them anyway. Same goes for T-shirts and other articles of clothing a line might not allow in the dining rooms at dinner. I don't hold the cruise lines blameless, either, if they look the other way.
We enjoy dressing for dinner, and that includes casual nights. We follow the dress requests outlined by the cruise line and don't understand why it's so hard for others to do the same, particularly as it is stated explicity by the lines in its marketing materials, pre-cruise documents, and onboard. So, for us, it's not simply a matter of jeans or no jeans but respecting fellow pax enough to adhere to the dress codes they've expected.