There's a big difference, IMO, between informal sessions and more formal performances. I run a free-for-all session where people do occasionally bring a song-sheet, and I've no problem with that. We also have lots of laughter and fun.

But last night, for example, at a local acoustic performance, there were was a young, 3-girl group singing. Beautiful voices, nice choice of songs - absolutely no communication or eye contact with the audience as their eyes were glued to a communal book in front of them. I mentioned this (I hope) fairly gently to one of them afterwards, and she readily admitted that she felt unhappy with using a book and hoped to dispense with it when the three of them had been together longer.