Lotte Bailyn has an interesting piece in Quartz about how lots of people don't take very many vacation days even at companies that offer unlimited vacation time. Except she treats that as evidence that firms' generous vacation strategies are being implemented poorly. I'd say just the opposite—the firms that offer unlimited vacation do so because they're confident their employees won't choose to take much time off.
Take me, for example. Slate gives me a finite number of vacation days. But even if they got smart and gave me unlimited vacation days, I wouldn't drastically alter my vacationing pattern. After all, you can call them "paid vacation days" if you like but obviously nobody's paying me to go on vacation. Slate pays me to create value for Slate through a mix of attracting readers, being helpful to colleagues, and, in some larger sense, being part of the Slate brand. To the extent that I'm able to do those things, it makes sense to pay me. If I started taking half the year off, my traffic would plummet and I'd have a problem with the bosses. Not a "too much vacation" problem exactly but a "not delivering the goods" problem.
By the same token, right now I'm not expected to work weekends and in fact usually don't do any work on the weekend. But if I had a great idea for a newsy post, it'd be extremely foolish of me to simply refuse to do it until Monday morning. If I get it done and it's good, then that reflects well on me and it's worth my while to take some time out of the weekend to do it.
Obviously there are vast swathes of the employment landscape where this doesn't work. If you're a waitress or an ER nurse or a teacher then your job is to be at the restaurant/hospital/school during certain appointed hours so that people can get their food/medicine/education. People like that need definite schedules with defined time off. But if your employees have mission-oriented jobs then there's no need to mandate that they work specific hours of the day, days of the week, or weeks of the year. Their job is to do a good job, and giving them discretion about how much time to take off is a way of looking generous while knowing perfectly well that people won't actually take much time off.