I really cannot understand a band booking a gig if everyone is not available in the first place, totally understand using deps to cover unforeseen circumstances when is gig is committed to.
Our drummer always takes a couple of weeks holiday in August every year, this year he has not booked the actual dates as yet, I will not commit the band to any dates in August, even though we have the option of a very experienced dep, the band will never feel 100% tight unless we have all our regular members playing.
Our band just do a couple of gigs per month which is mainly for pleasure, being in a pro band, I can see the situation as being different.