[gargravarr]

I think it stems from the limit imposed by the number of days elsewhere; since the days don't carry forward (or if they do, not all of them), you HAVE to use them, which creates incentive to take vacations or you miss out.

And just like anything else unlimited, having it available without pressure to use it is kind of like a gym pass, you COULD go, but do you really want that hassle just yet?

[WorldMaker]

Supposedly "unlimited" vacation with an annual required contiguous minimum can work. "Must take at least two weeks off sabbatical at least once a year." I would imagine you have to be careful to keep that "required minimum" from feeling more like a punishment than a necessary pressure release valve, as with any "mandatory fun" requirement.