If the boss keeps sending emails on Friday there is no way that other people will really feel secure disconnecting.
Plenty of places with unlimited vacation cue or even pressure employees to take a minimum amount of vacation.
Edit: u/babycake's comment says it better: https://news.ycombinator.com/item?id=29879742
But companies know full well what they're doing. They know that if they replace guaranteed PTO with unlimited, not only does the employee take less vacation (more bang for the company's buck), but they also get to skirt around state laws on paying for unused PTO when the employee quits.
Plus, without any guaranteed PTO, companies can just decline your request for vacation any time. They can say they're in busy season, or they need office coverage, whatever. Whereas with guaranteed vacation, the manager would have a harder time declining those hours you earned due to bad optics.
Unlimited vacation without a guaranteed PTO base is a scam.