If you are going to insist that nobody ever perform a song in public until they have learned it so well and practiced so many times in private that they can guarantee they will not flub the lyrics or forget verses when they do take it public, there's going to be one helluva lot less public singing done, from the best singers as well as the lesser ones.

Some of us would much rather not take a chance on having a momentary brain fart (to use a technical term) mess up the flow of a song than take the chance that someone will walk out because we have a lyric sheet in front of us as a cue card in case of such an event.

And for group singing, especially when it's a sing-along, not a rehearsed choir, having at least a lyric sheet -- and maybe even the "dots" too -- can be absolutely necessary if the lyrics or tune is not really simple, repetitive, and/or predictable.