Arguably, the problem is that there's not enough "slack" in the system to tolerate someone being out. You'd have the same issue if the person on leave got hit by a bus instead---or just quit!

Plus, benefits are often "unfair" in that they help some employees more than others. A sickly coworker gets more out of the health plan than someone who is as fit as a fiddle. Not needing glasses, I get literally nothing out of half of the "vision and dental" package. The tuition benefit does nothing if you're unable or choose not to take a course.