People using that 3D plugin could probably use it. :D
Other than that though, I suppose that could be helpful in a game I'm making that uses wasd and the mouse, because when the mouse is outside the browser window the mouse coordinates stop getting updated. If the action is frantic enough, the mouse could go outside the browser window frequently, depending on how the user has the browser window arranged.
Other than that, it seems hard to anticipate what people might use it for, though. If it's not hard to implement, perhaps for that reason, it should be implemented anyway, to simply give devs the option if they want it?