Asking what it means for something to exist is a topic in metaphysics, which is why the OP wants to talk metaphysics. If I can hit something with a hammer, it exists in the most straightforward way we can imagine. But do abstract things like happiness or red exist without there needing to be something that's red or happy? Sooner or later you get to questions like whether certain things can exist (whether or not any do) or must exist.
Skeptics are not always as interested in philosophy as one might think. Skeptics are interested more in what we know and how we know it. That's epistemology, not metaphysics. So when you bring up the subject in a skeptics forum, the skeptic is likely turn first to epistemology and isn't very interested in the metaphysics of what merely can exist abstractly. What reason do we have to think there might exist something that doesn't depend on the physical world? If there were such a thing, how could we obtain evidence of its existence? Those are questions that skeptics think of first.
The parsimony argument is the best. "Thou shalt not multiply causes unnecessarily." What problem does non-physical existence solve? Why does it need to be there? The best theories emerge not when there is nothing left to be added but instead when there is nothing left to be crossed out.