Many coverage-related jobs don't require a person's full potential productivity, so the fact that their productivity goes down with hours worked is irrelevant. If a cashier at a low-volume store only rings up a customer every 10 minutes but you've decided that they need to stay parked by the register all day rather than doing other tasks between customers, it doesn't matter if it takes them twice as long to ring up a customer at the end of their shift as at the start.