One advantage of a book, though it's seldom used, is that you can avoid only being able to think of one song- the one you always do- when suddenly it's your turn. Sameness, whether from page or rote, is a dead hand on many folk sessions. Perhaps a compromise- learn your songs, but use the book (with a pitch/ key prompt in it) to ensure that you can choose a fresh song, remember how it goes, and avoid those embarrassing false starts in little Aled Jones pitch.