"As for mics and leads, surely there are enough stories of musicians being zapped on stage to suggest that if something goes wrong these can carry more than enough current to be a hazard."

Well, yes... but that would happen whether faulty or not - the killer is the faulty piece of mains equipment to which they are connected - or, more accurately, the TWO pieces of mains equipment between which the performer becomes a human capacitor. The ground on a mic/cable is only as good as the equipment to which it has been connected!

Low voltage equipment does NOT require a test, only equipment with a mains connector.

It is arguable that if the venue hasn't got a competent person on duty, they should keep their doors locked!!