"learning a song is like learning to drive a car."
This evoked a wonderful picture of somebody driving a car with a copy of the highway code propped in front of them!
In a way, the two things aren't that different - the objective of the driver is to get from A to B: the objective of a singer is (or should be) twofold; to recreate the song in order that you (the singer) get the maximum enjoyment out of is, and to communicate that enjoyment to the listener.
As a singer (at one time) I cannot conceive of being able to enjoy a song while being so worried about remembering the words, I need a crib sheet in front of me, (remembering words is a technicality, not part of the re-creation).
As a member of the audience, if I see a singer has to have a prop in order to get through the song my attention will be diverted to "willhe/she, won't he/she", thus preventing me from enjoying it.
Jim Carroll