"In the many cases, the poet will probably know the poem "off by heart".
Not necessarily so the singer although they mays till use the song sheet as a prop even when they don't really need to do so."

Do you have hard evidence of those two propositions? Why the assumption that the poet knows his material, yet the contrary assumption that the singer probably doesn't? Sounds like a double-standard is operating there. Or a complete illogicality.

And why is it 'acceptable' for a poet to have a crib-sheet, but not a singer (apologies if I'm misunderstanding you on that).