The high costs of hiring and nature of specialization in certain industries makes it impractical to let employees determine how much time they will work each ye

The high costs of hiring and nature of specialization in certain industries makes it impractical to let employees determine how much time they will work each year. Now, if a new position opens up at a company and the fixed costs of bringing on new employees turns out to be low, and there are two equally qualified candidates who would like to split time with each other, then there's little reason why the employer shouldn't take them both on. But getting back to the point, doesn't it make sense for the employer to pay these two employees the full amount for the time they are actually working, rather than paying them half for when they work, and half for vacation/time off?