As Kat pointed out, choirs use music folders or books too, but they don't bury their heads in them. They know the songs but the printed sheet is there as an aid, as needed. Being able to glance at the sheet can prevent brain farts and their consequences.

I find if I have a song sheet in front of me and I know the song, I often don't even need to look at it. Just knowing it's there tends to make me less likely to go blank on a lyric.


Songbob, I agree about the tyranny of everyone having to stick to one particular printed version of a song.   Especially when your preferred version isn't the one in the book or, worse, when the book has some lyrics or chords WRONG. (I'm talking about you, RUS, for one.) "Version creep" can really throw a monkey wrench into a group sing when lyric sheets aren't used.

Acorn, I too prefer someone to use a lyric sheet or crib note than to leave out verses or mix up the order (in songs where that matters) or just bumble through.    And a book can be distracting if people are flipping through it when others are singing or when it comes their turn to lead a song.


Nick, the easiest and fastest way for me to learn a new song - to perform it in front of an audience w/o notes - is to have the lyrics (and maybe chords) in front of me in very large type the first 2 or 3 times I do it in public.    All I do is glance occasionally and briefly at that sheet while I'm playing and singing the song, and by about the 3rd time I don't need the cheat sheet anymore.   (If I don't sing the song again for a while - e.g., a song I do only at Christmas time - I may need to use the cheat sheet one more time to refresh my memory.)   
I think one reason people who use RUS tend to bury their heads in the book is that the type is so small and the format isn't easy to read (with lyrics jumping from the bottom left to the upper right of the page, for example).   If the print on a song sheet is really large, it's easy to read at a glance.
When I use RUS as a song sheet, I do NOT tend to automatically memorize the song the way I do when I use really big print song sheets - probably because so much of my attention is required to follow the format of RUS and read the mice type.