We all know that starships in Star Trek have artificial gravity. They also have inertial damper. Therefore, we can conclude that they have the ability to counteract gravity. However, as those technical manuals of Star Trek tell us, starships are using impulse drives for slower-than-light travels, and those impulse drives are traditional fusion drives which use reacting forces to create thrust. Now here's my question: since we have the ability to create anti-gravity field, why not use anti-gravity thrusters instead of those fusion drives? Anti-gravity thrusters are much more agile than normal thrusters, obviously.