Cluin: Recording studio? I wondered if anyone would mention that.

I use music and/or a stand:
(1) very occasionally when accompanying a song that I haven't learnt yet, in an informal session where it's not a proper performance.
(2) in concerts with a complex schedule and smart timing required between items like Martyn Wyndham Read's Song Links concerts and Maypoles to Mistletoe - you need to have the running order in front of you to know when you're on next and what instruments to have ready, also possibly as a prop if you've had to learn a song for the occasion which you wouldn't normally have in your repertoire. In "Song Links" John Kirkpatrick had the words in front of him e.g. for Moreton Bay, which is a long song with incredibly flowery lyrics, but we noticed that when he performed the same song in a solo set at Sidmouth last year he'd learnt the words - no music stand in sight!

(3) In a studio for sure! In my experience of recording there's a lot of stuff done at the last minute. I've had to sit down and play a cello part for a tune or song I've barely heard before. What works for me is to try things out, then when I hit on something that works scribble it down on some manuscript paper before I forget, then use that for a take. Even writing down the melody helps me to look ahead and plan an improvised bass line while I'm playing it. In a recording situation, precision is more important than spontaneity and I can play far better if 90% of my brain isn't desperately trying to remember what's going to happen in the next bar...
In the "Windsor Terrace" track of Sharp Practice Gina is playing recorder from dots I'd scribbled out about 10 minutes beforehand - a run though sight reading, slight debugging of my dots, another run-through and a couple of takes and that was it. Musicians who can read that well are a terrific asset when recording, though the downside is that good readers often find memorizing hard because they usually don't need to.
So my recording studio "must-have" list includes stand, manuscript paper, pencils and a rubber (um, that's eraser for our transatlatic friends...)

(4) in a ceilidh band. I don't like this but while there are tunes that other members of the band have introduced and which I haven't learnt yet I sometimes need it. I'd MUCH rather be watching the dancers and keeping an eye on the caller.

The rest of the time... I have no time at all for performers who rely on the words in front of them to such an extent that if you snatched it away at any moment they'd never know how the next line went, or who waste minutes shuffling through the book looking for something, as described earlier. I've never heard a good performance from someone who did that.

Anahata