Some people do like it, and I'm glad it's working out for them--I really am.

But adding all that manager and corporate discretion sets one up for abuse when things go wrong at either the manager or corporate level. For some, I suppose the benefit is worth it--but if "unlimited vs set days has almost no bearing on how many PTO days someone will actually take", then people are giving up a lot of guarantees for very little benefit on their side. Especially the payout when you leave.