For me, there is a very important distinction, regarding the purpose of the singing: Is it to entertain, or is it to share? Accordingly, the singer becomes a "performer" or just "a sharer". The two have different requirements:

a) At gigs, or when supporting a guest at my home clubs, I take the view that the paying punters want to be entertained, and I only sing songs I know well, where I can concentrate on the performance.

b) At singers nights, especially at my home clubs, I often bring out new songs for their first airing, and to get initial reactions from the friends there who have supported me so much in my songwriting; there, I am "sharing" my latest song, which I have invariably not learned by heart yet (and may never do, if the reception is not such that would make it part of my "main" repertoire).

Also at singers' nights some singers try to be topical, and wish to sing songs that they only sing once a year. I have no problem with them using a book; they are "sharing" the song with me, not trying to impress me with their performance of it.

I do find it distracting in a concert when the performer uses a safety net. But when big names perform on TV with autocues, I cannot chastise the "sharers" using books in singarounds. And I would even argue that the latter approach (the sharing) is closer to the folk process, safety nets or not. But in the end, it's horses for courses - entertaining or sharing.