As Mr Gnome pointed out , there is a VAST difference in having the words (Or Cue Cards) handy , than Singing AT the book , holding it between you and the audience !There are two local to me clubs I have stopped going to because so many of the 'floor singers' did just that the last time I visited .
Personally , I think that you cant do a song full justice unless you KNOW the song , though I accept that a lot of people dont have as good a memory for lyrics as some of us .