don't have a problem with people being unavailable, our drummer likes a weekend off every month but as long as they're in the band calendar with plenty of warning it's not a problem, I try and book holidays when other band members are off but there's one week in they year where I have no choice, but it's on the calendar 9 months in advance.
But it all boils down to how much people want to gig I suppose.
edit, and pressure from wives and partners