I've just read through this (revived) thread and have these comments:

1) I agree with Art T, that it is extremely annoying to be in a singing session (workshop, singaround, pub, whatever) and have other people thumbing through books instead of listening -- people run through songs in their heads instead of listening too, but at least you can't hear them doing it. So, books need to be used with etiquette and discretion.

2) In my experience, the "rail song followed by a rail song" is just one of those things that happen in small sessions when singers are listening to one another closely and the songs themselves spark memories of other songs, linked by melody or theme or even the singers we learned the songs from. In my opinion, this is part of the magic of small sessions with committed singers -- and this kind of magic can be obliterated by those who insist on singing their party pieces or their entire repertory from their personal books: I was recently at a late night session where someone entirely killed the session by presenting piece after piece, handing around lyrics, and totally ignoring the vibe of the session. The rest of us quickly developed the need to get some sleep and left.

3) My problem with the infamous "Blue Book" is that people get married to the version presented therein and stop listening to singers presenting different versions of the song -- this happens a lot with traditional material that has been "fixed" by the blue book.

4) I am one of those singers with a prodigious memory, slipping a little as I age. I am often asked to prompt other singers -- and sometimes do it unasked when someone is flailing about for too long. I do know that some people need aids, which is fine -- just please be subtle about how you use them, use only when needed.

5) As I have often stated elsewhere, I make a distinction between traditional material and songs by known writers. Traditional material is fairly fluid and exists in multiple versions and will stand massive variations and still live -- you don't have to get it "exactly right" because there is no "right" version (although you should try to sing a song with respect for the song, the tradition and your listeners). When you sing songs by songwriters, contemporary or otherwise, please make an effort to get the words right, in the right order -- and use "cheat sheets" if you need to do so, after making an effort to learn the song correctly, of course. As a writer, I hate it when people muck with my lyrics or the order of my verses.