I'm in lots of bands/dep occasionally and the golden rule (as others have said) is:
If you've said yes to a gig and it's in your diary, you do it.
I've just had to turn down a well paid wedding gig as I'll be up in Manchester (playing for next to nothing - an old mate's originals band) next weekend. This gig was organised months ago, so I'm doing it.
If you start ducking out of gigs for better paid ones, people will quickly stop asking you to gig... you're only as good as your word.
Use Google calendar and encourage everyone else to - this makes everyone's life a LOT easier and cuts down the potential for oopsies in my experience.
As someone above said - if someone in your band is getting offered stuff like IOW festival.... that'd be a different matter.
I'd imagine they've made that pretty clear from the outset!
And as someone else said - get a few deps on board!
As for people turning up to gigs/rehearsals without having practiced... that's just laziness, surely?!