Sorry to resurrect an ancient thread, but I've been musing on this topic recently, partly to understand my own behaviour. By way of background, I only sing in 'informal' sessions.

With one exception, I never sing from a book. Broadly my rationale is that much of the life of a folk song depends upon your own interpretation and variations, and having a fixed set of words as a 'definitive' version limits that. As I normally sing solo, there are no issues agreeing what to do.

Carols, on the other hand are essentially based on a group of singers, and so everyone needs to agree on the same lyrics. For that reason, I almost always use printed words with carols - even when I am singing them on my own.

Last night I sang about six carols at a session, all from a book and all on my own. I'm not entirely convinced that my rationale makes sense in this circumstance.