I have said this often, and I hold my hands up again, I enjoy all kinds of singing, from the book, made up as you go along, and"folk processed" but I get embarrassed when folks forget the words and are scrambling about trying to remeber them in mid song, especially if it is a "serious" song, during a session,and In my experience this is FAIRLY common and I personally, would rather use, and see others use, a song sheet or book, rather than destroy a carefully crafted song(lovingly made by some songwriter) by getting it wrong, or breaking the mood in mid song by forgetting the words.
Most of all there are few things more embarrassing than forgetting the words of a song YOU WROTE!!!! (and this is not that uncommon,) as one sometimes switches to an earlier draft or includes a verse that was removed!!)