Predictable... There are cameras with interchangeable backs, and cameras without. Both were manufactured through decades of popular professional, amateur and casual use. And dozens (more?) of lovers of each and both.
So we concluded some people like them, some don't, and some do some of the time and some never.
And like whenever there are multiple (value-free) opinions, some have to tell others they are wrong.
If you treat this as accounting, there are business cases for wasting film or not. I prefer not to for purely aesthetic reasons, just as I don't like leaving tasty food on the plate, even when I'm full or have nothing to take pictures of. Not always good for me, but that's the personality I have.
As for backs, I have maybe a dozen functioning backs for my three Kiev 88 bodies. Actually, this morning's activity is to look for signs of light leaks on a couple of these.
Usually one is loaded with B&W 100 or 400 (most often Foma), one with color (I have five rolls of Velvia left in the freezer), and one with IR. Right now one is loaded with Washi ISO 3, waiting for a waterfall or something like that. And there are 70mm and 220 backs too, but they fit into those categories.
And as far as film being cheap, that depends on what else you compare it to in your practice. My gear was paid for 20 years ago (mostly), and CLAs are infrequent. I live where I can drive an hour and see almost any landscape I want to photograph (except oceans!), so film is more expensive than gas or kwh. My time is more expensive than the film, but shooting often goes in the "health maintenance" column too.