Howard: [[Genie said in a post yesterday that he would rather hear a good singer using a book than a mediocre singer without one who forgot the words. ]]
First, I'm a "she."
But did I really say that? If I did It was a slip of the fingers.
I would rather hear a good singer singing from a book or song sheet than a mediocre one (and I'm not necessarily referring to vocal quality) delivering a song totally from memory.

I'd add that if the song is good, I'd rather hear someone sing the lyrics correctly with the aid of a song sheet than hear some of the best verses made up or mediocre lyric lines improvised because the singer, who really did know the song, had momentary memory lapses.

I think the issue of whether you should prepare, rehearse, and memorize your songs is one issue. (Except for very informal song swaps, that's pretty much a given.)   Whether you should ever have sheet music or cue cards, etc., as a PRN memory aid during a performance is a different issue.   

Lest my earlier posts may have suggested I usually perform using books or lyric sheets, I don't -- though I sometimes hand them out at song circles and workshops, especially when I want others to sing along.   (And I do think there are many songs that are far, far better with harmony all the way through and may not even have a chorus.)
The point is that between my professional music gigs and my participation in unpaid jams and song circles, there are well over 1,000 songs that I know by heart (many more, if you include songs that I don't have to back up with my guitar).    Many of these songs do not have a chorus and some have lyrics that are more easily mixed up than others.   I often have to "shift gears" from one day to the next and do an almost entirely different playlist. (I'm doing some sort of theme program for a few weeks and need to switch to another very quickly.) I also jam and share music with different groups, each group having a kind of different repertoire of shared songs. During the course of a year, I really NEED to perform at least a couple hundred songs and be prepared to do hundreds of others upon request (or - to go back to Mary G's initial point - to participate in a song circle that has a specified or unwritten theme).

Pip, several people have suggested that most folk songs have only a few verses or have choruses for people to sing along with. Obviously there are many that aren't that simple.
You say you'd never use a crib sheet to sing 24 verses of Little Musgrave.   But if you wanted others to sing along with you, would you?   I gave "Lorena" as an example of a song I love to play and sing but would just as soon have others sing along with me.
(In jam sessions it's not unusual to have 8 to 10 instruments playing at once. Even if your voice is strong, it's hard for one voice to be heard over 3 guitars, 2 banjos, 2 fiddles, a stand-up bass, an autoharp, and a mando. Having others sing along may be the only way for the vocals to compete with that.)   No matter whether I need the lyric sheet or not, the others usually do. And when I'm asking others to sing along, I don't have the luxury of ad libbing a lyric and having nobody notice.

I don't think anyone here is a fan of having people bury their heads in books or lyric sheets. I think we pretty much agree that if you have to make much use of your printed page it's probably going to make your delivery less powerful and effective.   But if a symphony orchestra or choir can bring you to tears while using sheet music on stands or in folders and making eye contact with the conductor choir director -- not you, the audience -- how can it be that a solo or duo or trio couldn't thrill you and move you if they happened to use a printed page in similar fashion?