The Scourge are great anti-air units. They fly themselves at enemy ships, doing 110 damage when they do. A fleet of 12 or so can easily take out a Battlecruiser, making them devestating to both an army and to an economy. They come out 2 at a time, allowing you to create a fleet very quickly. They have no defense against ground attacks, however, so keep them in your base until you are ready to use them.