I don't know <I>why</I> this is such a recurring topic, and a controversial one at that!
DCL very <I>clearly</I> states (in their brochures, website FAQ, and the "Passport to Magic" or Wonder booklets - as well as part of DCL "Cruise College" for "Disney Specialists") what you should and should not wear to dinner. There will always be those who prefer to defy the "recommendations" (aka "dress code") and see what they can get away with. But threads on this topic get so hot, they should probably be on the debate board.
