What is Modal Logic good for?

Over this break I have been attempting to learn modal logic. Modal logic is the symbolic logic of necessity and possibility. So, for example, its possible that a Republican will win the White House this year. Anything that doesn't entail a contradiction is a possibility. Something is necessary when its denial produces a contradiction--for example: A square necessarily has 4 sides.
So, what is Modal Logic good for? Is it just some garbage made up by philosophers to keep the publishing going and get some professors tenure? Well, yes and no. Modal logic is important to philosophers because a large chunk of the last 100 years of philosophy has been something called conceptual analysis. Conceptual analysis is the attempted defining of a difficult term by its necessary and sufficient properties.
For example, one huge area of this type of work is the conceptual analysis of 'knowledge'. Modal logic helps us to precisely ask questions like "Is it (logically) possible for a person to have knowledge without being justified?", "Is it (logically) possible for a person to have knowledge by accident?" etc. etc.
This work also has certain implications for philosophy of science--are laws of nature necessary? etc.
So I would whole-heartedly agree that modal logic is very important for conceptual analysis. But modal logic is also used for at least one other purpose. Philosophers of mind have been using modal logic in the past 40 years or so to make metaphysical arguments. At first, I didn't even question. Probably the most famous metaphysical modal arguments are Kripke's in "Naming and Necessity". He argues against physicalism in that lecture. I don't want to get into the argument, though its very interesting (and to this day, I am undecided whether I accept it or not. I've gone back and forth for the last couple years). My point is--can modal logic tell us anything about the way the world is? Its hard to believe it could. Modal logic is seemingly a priori. So all bizarre and controversial exceptions aside, Kripke's argument seems to be saying that physicalism is necessarily false in any world with consciousness. And that is a tough argument to make when you put it that way. For I see no inherent contradiction in a physicalist world with consciousness. And I am a dualist, so it isn't just dogmatism going on.
So here is my challenge, to myself, and to anyone who wants to do some preliminary research on modal logic: Give me a simple modal argument which proves something metaphysical. If someone can prove that metaphysics is possible using modality, then I will be more open to the more controversial arguments.