I agree totally with Captain Swing - reading from crib-sheets is an insult to the audience.
In all the years I have spent with traditional (source) singers, I have known very few who have use written texts while they were singing.
One was/is a local singer who now has Alzheimer's, but is still asked to sing occasionally and has been encouraged to write his songs in a book by his daughter.
On the couple of occasions I have seen it happen elsewhere, the singers have apologised for having done so and have obviously been extremely uncomfortable.
Walter Pardon did it once; he had put a tune to Thomas Hardy's poem, The Trampwoman's Tragedy but had stopped singing in public by then so had not bothered to learn it, so he read it from a sheet in order that we might record it. He would never in a thousand years get up in front of an audience to sing from the page.
I believe it is essential to know the song inside out before you can make a good (or even passable) job of it, otherwise you end up just singing the words, and singing should be much more than that.
In my experience, reading from the page has always made the song stilted and unconvincing.
Risking the wrath of god (or Khan) being brought down on my head, it is why I have never really liked The Coppers singing. The fact that they read the text from a book has always made their songs poorly phrased and samey; the great exception being Bob Copper's solo Topic album (Rosebud in June??) where he proved what a fine singer he really was, to my satisfaction at least.
Jim Carroll