I'm with the 'from memory' set except, as someone mentioned above,
when the mortal flesh has intervened such as from a TIA or just plain
old age. In that case, the singer isn't cribbing it because s/he doesn't
know the song well enough, but rather because of a memory issue---
which means they understand how well one must understand a song to
sing it in public.

The other thing about singing from memory is that if you make
it a rule then you'll be singing the new song constantly. Driving, at
work, out loud and in your head/heart. Which means that you go about
with a song on your lips or in your heart much of the time, which is
a good thing, now isn't it?

Unless of course it's Barnacle Bill, and you're at work.

And forgetting the words isn't that awful, really. Hell, if Lou
Killen can do it on occasion, I sure can. Only thing I can't figure
out is how in the hell he gets the words projected up there on the
ceiling where he always seems to look for--- and find--- them. I
can't see him. Must be some sort of invisible ink laser projector
he carries with him, with about 10,000 songs in its memory.