I don't think it's hard to find people who can be unprepared for doing anything on the night but can put in a good floor spot. They will have experience on their side though.

I've seen the type you refer to although not that often. Saw one last year in a folk club that was just taking turns round in a circle so it was clear when his turn was coming. Still it was tune guitar, get book out, debate which song, start off, wrong key, start again, get stuck, try different song. I can't see an excuse for it.