This is a good idea. I've had two employers with "unlimited PTO". One of them explicitly set an expectation that I was expected to take at least 4 weeks per year. And it wasn't in writing, but everyone was told this, so there's "witnesses" which is better than nothing. This actually worked really well, and everyone seemed to take an appropriate amount of time, but no one was explicitly keeping track or expecting you to fill out paperwork just to take a break.
Another employer said you needed to talk to your manager and justify time off with regards to results, and the timing of your break, relative to releases, etc. In this scenario I hardly took any time off, but folks who clearly didn't care about their manager took SO MUCH. Learn from me, though: they never gave me the promotion I was chasing anyway.