It's part of the very idea of the SI system that a single unit is used
for each physical quantity, in a unified manner


This is fine and well while you're sitting in an armchair. But in the real
world there are sometimes compelling reasons to do something different from
the way a machine might handle things. In the case where

1: Not much interfacing with other units is involved
2: Rapid and accurate organic processing of the numbers is essential,
sometimes in adverse conditions.
3: Communications is suboptimal
4: A narrow range of values is involved

I'd say that it makes sense to use whatever units are most convenient in that
case, not whatever would make some world standards body twinkle its toes.
Altimeter settings are such a case.

Jose



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