Compare the cost of keeping one surplus pilot forever deadheading around the system to the cost & disruption of canceling (say) one flight a month because "a pilot got sick, it was not legal to fly short-handed".

Having a (relative) bunch of surplus critical-task workers, forever being shuffled to where they system guesses they're most likely suddenly be needed, makes perfect sense.

(Yes, you'd have to be a bit more sophisticated, so your surplus pilots got enough flying time to stay certified. And didn't get pissed and quit. Assume that I know why RAID 5 is better than RAID 4:)