I sing in a folk choir and we have a rule - learn your part and no crib sheets in performance. I greatly approve of this because I think it helps us to improve communication with the audience. The group that I sing/play with has the same rule for the same reason. However, I see no problem with a performer using a crib if it helps them to turn in a good, entertaining performance. I avoid sessions where players depend on books, folders etc. They screw spontaneity.