Among a certain set of Silicon Valley companies, unlimited vacation time is a powerful weapon in the perks arms race. Netflix, VMware, Evernote, and Palantir all offer the practice, which is attractive to would-be employees and a system somehow in keeping with groovy contemporary ideas about performance and work/life balance.

But by now, many articles like this one in Fortune have cast some level of doubt on the system. It's difficult to implement, for starters, and studies show workers don't necessarily take any more time off than they otherwise would.

“People always think unlimited vacation is such a wonderful thing, but when you really study the impact, you see that on average, people aren’t really much better off than the ones who don’t have it.” or so Bettina Deynes, vice president of human resources and diversity at the Society for Human Resource Management, tells the Chronicle.

While an abundance mentality might be pleasant, it's that very perception that's probably the biggest perk of all. Consider the unlimited options we're commonly used to: bottomless mimosas, or bread sticks without end. There are limiting factors — often cultural or gastric ones — that come into play.

"We don’t work in a 9-to-5 world anymore," Katie Denis, senior director of vacation advocacy group Project: Time Off, tells the Chronicle. "The office is omnipresent, so trusting your employees to take time off when they need it and work when they need to work is a great concept, but it doesn’t actually work that way... On the whole, in almost every instance when we talk to employees, they want boundaries. They want to know the speed limit of the road they’re driving on.”

Finally, as the prevailing winds begin to shift in Silicon Valley, another important factor to consider is layoffs. On this subject, the Chronicle turns its gaze to LinkedIn, recently purchased by Microsoft and probably due for some downsizing, and Zenefits, which is reportedly making another round of cuts. In the standard system, unused paid vacation time would be added to one' severance. However, if every workday could be a paid vacation day, that system breaks down, and no unused vacation days are compensated. "The biggest benefit in offering this for the employer," as Deynes puts it simply, "is they don’t have to carry all that unused time in their balance sheet."

Related: Layoffs At Tech Companies Double What They Were Last Year At This Time