The original post was about singarounds in the UK folk scene. And yes, these are usually round-the-room with each person taking a turn to sing. The rest of the audience will join in the choruses, and sometimes if a song is well-known they may join in the verses as well (not always desirable if you're trying to sing a different version). Group singing as such is less usual at these events, in my experience, but at that type of event then shared songsheets are probably acceptable, even necessary. However I interpreted the OP to mean a solo singer taking their turn.

I still fail to understand why it should be regarded as too difficult to prepare two or three songs ready to be sung without the need for the words in front of you. Most folk songs are quite short, after all. In most cases you go to the event expecting to sing, and you should prepare and rehearse beforehand. Or is that too much to ask?

As for the longer songs, if you can't sing, say, Tam Lin without the words in front of you then imo you shouldn't be singing it at all.

I wonder if some of the singers who rely on books are setting the bar too high. If you expect to have 50 or so songs ready to performance standards the whole time, or not ever to repeat a song over a 12 month period, that's asking a lot. Most performers, amateur or professional, will have (I would guess) not more than a couple of dozen songs at most worked up to performance standard at any one time. They will know many more, and they'll be learning new ones, but they'll be resting those and they won't be ready to perform. Some they probably could nevertheless perform with little or no rehearsal, while others will require more work before they're ready to take out.