Parkfell, apologies to be pedantic, but the LST on the A320 gives a MPA/IR (which as you correctly said is type specific) the IR on a piston twin is a SPA/IR.
There were a handful of old turbo-props around with only a basic alt and heading hold autopilots where all approaches has to be hand flown, and just because the approaches were hand flown it still didn't give SPA IR privileges, the demarkation is simply the fact one is multi crew one is single pilot. I think the key point on a MPA type some one is helping without half the workload.
So I agree the automation is a very valid practical reason why a MPA/IR shouldn't give full cross credit to a SPA/IR , as far as I remember an IR on a multi crew type has never inferred full cross credit to a SPA/IR irrespective of the automation level.