Generally true with respect to ordinary OS tasks. Often though you want to respond to specific events within a fixed time limit or always do X at interval Y. Neither things using all the CPU resource, leaving gaps to fill. What you do the rest of the time is low priority things that don't matter them not happening bang on interval Yn to within nanoseconds.
That is my experience from automotive engine controllers. On those we do lots of low priority things. The issue that comes in to play is testing and validation. If you are running other tasks on a controller with safety critical tasks generally you want to test everything to the higher standard if you are mixing on a shared host.
Related to running something like Linux as a low priority task under a hard real time system gives the argued (I have my doubts) ability to sandbox the none safety critical tasks so that they can't do things to interfere with the safety critical portion.
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