Wikipedia has a decent entry on the Barcan formula. It is generally held to imply that nothing exists contingently, and that in turn is generally thought insane. I would seem to be a good example of something that exists only contingently. But there are some people who think the Barcan formula can be defended, and it would be nice if it could because it makes certain aspects of modal logic much easier than they otherwise are.
That said, I am finding it hard to imagine why the Barcan formula and its consequences would be relevant here. If you believe that God exists contingently, then you think the Barcan formula is false. Since not many people accept it, that isn't much of a loss.