Or, for example, if the employee was an orthodox Jew who requested not to be scheduled on Saturdays?
I worked in-house (as a freelancer) at a studio for a while where one of the artists was orthodox. He made a deal with the employer where he left every night before sundown he never worked on Saturdays. This was a broadcast studio where you could had frequent short deadlines and many of us had to work late into the night and through weekends. I was surprised that his coworkers didn't seem to have problems with it when they were digging in for a late-night crush and this guy was packing his bags at 5 pm. Why would they? The studio made the deal because the guy was very talented and did great work during his regular hours.
I've had similar situations working at law firms that hired contract attorneys. Often they're young mothers who want to keep working, but only 9-5 and sometimes just 3-4 days/week. That can be a little irritating to the regular attnys who are working 60-70 hrs./wk, but everyone understands that it's the deal the associate and the firm cut.