A reaction drive's efficiency as a weapon is in direct proportion to its efficiency as a drive.
The Kzinti Lesson (above) is the lesson the humans taught the Kzinti in Larry Niven
's short story 'The Warriors
'. The bottom line is that the better a starship drive is, the better off you are mooning the enemy with your ship and using it to blow them away.
The Kzinti, an alien race, invade the solar system, having been told by their psychic spies that Earth was fully demilitarized. Humanity, however, surprises them. They don't have weapons, no, but they do have powerful photon reaction drives and destroy the Kzin fleet by using the exhaust as a weapon.