In two previous jobs I've had "unlimited PTO" and almost everyone in the office worked more than if they'd been given bank holidays. Pretty sure it's just a mgmt strategy that sounds awesome on paper to workers and is known to actually reduce time away from office.

That said, I personally made good use of it. Had a coworker or two ask about how I thought it was "fair" to the company. I knew the rate at which my (salaried) time was being billed to customers and didn't see it their way at all :P