Ours (secondary) are almost invariably in holidays, with teachers giving up their time.
I'm running one at Easter which involves us leaving two days before end of term - it's the only time I've ever known this happen, & it's because of logistics to do with the destination we're going to.
It's a bit of a ball-ache actually; I've spent the night doing my KS4 planning & I've had to factor in two days of work that a cover supervisor can reasonably deliver. I've got three classes heading into exams who could do with me teaching them for those lessons.
Also, we do do the occasional Friday-Saturday in the UK for theatre trips & the like. They usually run once year 11 have left so that we can deploy staff to teach those left in school with minimal disruption.
The argument about paying for school jollies & students from less well off homes being unable to go is a bugger.
All I can say is that we have a system whereby Pupil Premium money heavily subsidises places, & there's always a flexible payment system in place. It's by no means a perfect set up, but the only time I can recall someone actually being refused a place is when they'd already had several trips subsidised & the parents had invariably failed to make the payments they'd agreed to.
One argument in favour would be: say one of my own dc is desperate to go skiing. I certainly won't be booking a family skiing holiday - can't afford it, & don't fancy it anyway. I'd much rather save up for the dc in question to experience it via a school trip.
Or alternatively just tell him/her no chance, better things to spend my hard-earned on. IME teenagers are quite stoical about that sort of thing.
...& finally, the 'no authorised holidays' thing certainly isn't something teachers asked for! But I'd agree with you that it doesn't sit well with extended jaunts out of school in term-time that take out a substantial proportion of a year group plus their teachers.