Pleural pressure is negative because both the chest wall and the lungs are pulling it from opposite sides. You have to remember that negative doesn't mean negative pressure. It means the pressure is negative compared to atmospheric pressure. So, anything below 760 torr would have a negative pressure. Technically yes, pleural pressure does assist in breathing, but try not to think of it that way. Think of the pleural space as two lab slides stuck together with a drop of water. When you lift the slide up, the other slide sticks to it.