See, I can't help but worry about these absolutes. Yes, it's nice to get up and sing without any psychological props, like wordsheets, etc. But then you look at other styles of music (say, oratorio) where the holding of the book is part of the form no matter how well you know the piece. It doesn't really prevent communication. And sometimes an otherwise excellent and well-rehearsed singer gets a blind spot about a particular set of words ... isn't it better for them to have the words to hand just to stop them from going to pieces when you know they're going to really deliver as long as they have that security there?