The question is, how fast can the Enterprise execute a dive or point its bow up? Why couldn't Kirk dodge a long-range torpedo by dropping instead of trying maneuver left or right? Consider, also, the advantage in coming in on an attack from an above angle, with your bow pointed straight at the enemy. He would have to point his bow up to fire at you, or try and go directly underneath you!
I guess the question is, why doesn't Kirk pull a Wrath of Khan