Is setting the time in advance for a performance a necessity?

When we have the occasional Open Night at out folk club, nobody knows when they are going to perform their two items until the compere tells them. Nobody arrives late as they might miss their chance. If anyone left straight after performing (which, in fact, does not happen) they would not be considered in future, Some are looking for a gig which means that that they try to perform their best and are polite so as not to spoil their chances.