> It makes me wonder where the divide is.

I think it works beautifully if you're applying it along with "refactor mercilessly", backed by good unit tests. It's a principle that doesn't necessarily stand on its own; it needs to be part of a healthy, iterative system of development.

If refactoring becomes difficult or dangerous for whatever reason (eg, designing an API for long-term, widespread usage), then YAGNI may not work.