There’s obviously the benefit that they don’t have to pay them, but surely schools want someone with an IR to instruct at least?
You certainly don't need an IR to be an instructor. To teach PPL, it's all VFR, so an IR will never get any use. I was a PPL instructor for two years before I got my IR, then another year before I moved on to an airline job, I can't say there was any difference in my instruction before or after I got the IR. Especially since most basic training aircraft aren't fully equipped for IFR, so it's not like you'll be going above the clouds every lesson, as you'd have no way to get back.
What exactly are you looking to do? Full time PPL instruction really doesn't pay very well, so isn't a long term career prospect, and IR instructor is now going to be very difficult to get into without airline experience.