Back to the point about dressing for the theater. My belief is that if you dress appropriately for an occasion, you are more likely to behave appropriately DURING that occasion.
Example, you say?
Well, if you are dressed for the theater in DECENT clothing, you are less likely to put your feet up on the seat, take your shoes off to examine your feet, eat candy and let it spill all over your decent clothes, or lounge around and talk conversationaly to your friends.
If you arrive to the theater in the clothes you wear to say, clean the car or sit in class all day in, you are more likely to feel like you can just sit back, put your feet up, slip off the flip flop and examine yourself, eat the candy and hell, who cares if some of the chocolate melts into the clothes!
And I say again, you wouldn't show up to a beach party in evening wear and you wouldn't wash your car in a nice outfit, so why do you feel it's ok to show up to the theater in clothing you would wash the car in?
And to the point of whichever poster said "not everyone has paid $100 for their ticket", it's not the price you paid for the ticket, it's the experience of seeing a live performance that should count for showing some respect to not only the performers, but the patrons around you.
"My dreams, watching me said, one to the other...this life has let us down."