It would have to be very bad for me to cancel but certainly there are limits. I don't think any organizer wants the singer or musician to collapse on stage. I guess the other thing is if you really have laryngitis and you're the singer there might be no way you could do a gig. How much croaking could people put up with. Although I've managed to get through many a set with a sore throat, and if my voice couldn't hit some of the notes I "talked" my way through the verses. These were just pub gigs - not concerts. Like any job - attendance and reliability are important so I think the advice would be only cancel if you absolutely have to.