VT, I think most people accept there are circumstances when a songbook is necessary, perhaps because of genuine memory problems or because an artist has been called upon to perform a particular song at short notice.

Where it should be discouraged is where there are no good reasons, where a singer has simply said, "Oh, I can't remember songs" when there is no medical reason why they shouldn't, and what they mean is they haven't put in the time and effort required. In my experience, I'm afraid, that's nearly always coupled with a poor performance.