The situation in question is something that occurs at Moor and Coast Festival, however it is usually because artists are genuinely busy (and Glen's time organisation leaves a little to be desired sometimes! (A Joke Glen, I promise!)).
Last year Emma Heath was doing a concert spot at the school which ran over time but she was then supposed to be doing a spot at the Endeavor just 20 mins after the concert finished, so in that situation, I think it's understandable that you wouldn't want to hang around. I for one would probably be hunting for food!
There is another side to this old penny, some people find it an annoyance that guest spots are scheduled at all in Singarounds, for them it is an annoyance full stop.
Sometimes it works, sometimes it doesn't and sometimes people's etiquette leaves a hell of a lot to be desired.