^^I wouldn't think that the modern 3D glass would cause appreciable distortion while watching a 2D movie. They are only polarized, and even though they have a slight tint, they do not noticeably make things darker (maybe very, very little).
Even though we were told when to put on our glasses during that showing I was talking about above, I think that was only to inform us when the 3D portion of the previews/movie was coming -- not that we couldn't
be wearing our glasses during the 2D. Some people may watch those first few 2D previews and not realize that they are supposed to be only 2D (although it should be obvious to most people).