Yes! Going to the theater should be an event- a lot of people have to put in a lot of time and effort to put on whatever play or opera or concert we go to see, so when the audience dresses up, it's like we're saying that we know how special that is. And definitely, if you're going to an older, grander venue like an opera house, it would just feel out of place to sit on a gilded seat in jeans! They originally built those grand staircases to show off gentlemen in tailcoats and ladies in ball gowns, so why should we ruin the architects vision? ;)