It depends. In an originals band I would say if someone isn't available then the band forgo the gig and look for another date, but when I played in a busy function band for a few years everyone had at least one regular dep. If someone's booked a holiday and a gig is offered on that date I don't think anyone would expect them to cancel their plans to play a gig, would they?
What really gets me is when someone pulls out of a booking at the last minute to play another, better paid, gig.