Howard: "" I can definitely recall one poster who uses a book who has a repertoire of more than 50 songs. Another justified using a book by saying they would not repeat a song during a year."

I'm not sure if any one person said those things as such. I, like some others, have said that some songs I sing are season-specific or situation-specific and not likely to be repeated often or soon.   And like some others, I've said there are occasions where I make use of song sheets. But I don't know that those of us who said we -- or our song circles - sometimes use lyric sheets have, said we are bad singers or don't take the time to learn songs. On the contrary, it can be precisely because you DO know many hundreds of songs by heart -- and perform them often in public, usually without books or lyric sheets -- that it's not feasible to practice all of them hundreds of times every month or even every year.   
But sometimes an occasion arises that calls for a song you didn't expect to want to sing at a particular song circle.*   If you happen to have that song in a notebook (which I've seen many people bring with them to song circles), I think it can be a better contribution to go ahead and do the song, glancing at your song sheet before or even during your performance, than to skip the song just so you can do one that you've practiced more recently.


*This often happens in the "piggybacking" phenomenon that Mary Garvey talks about - where, especially in "chaos mode," a song one person sings brings to mind another that you think would be an especially good fit.   There's a good chance you weren't anticipating performing that song and didn't rehearse it for the spontaneous, informal gathering.