"The faffing about with pages, usually means someone just sang that particular song before I got my turn and I have to find another, or else the one I had chosen to sing is not the most appropriate, doesn't fit the mood of the room or is just plain wrong for the state of throat."

But that's what I mean by preparation. If you expect to sing say 3 songs during an evening, don't practice only these but prepare a few more. Then if you have to change your planned repertoire for any reason you're ready with an alternative. No faffing necessary.

If someone have genuine memory problems and has to use a book, then they should learn to use it so it is merely a prompt and not (as so often happens) a barrier between singer and audience. However I doubt that most book users have medical reasons.

No one is infallible, and everyone makes mistakes from time to time and forgets words. It happens to experienced singers, including professionals, far more often than you realise, because they have learned how to cover it up. Most of the time you wouldn't know. This is another performing skill which cannot be acquired if you rely on a book.