^The way I see it, any ship entering a combat area needs to have a means of propulsion, and impulse power would allow the Reliant
to maneuver with something besides minimal thrusters.
Additionally, consider what Decker told Kirk about the redesign of Enterprise's
engine in TMP. Ships were able to increase phaser intensity by routing them through the main engines, so any vessel hoping to use phasers would need at least partial use of its main drive systems. With torpedo guidance unavailable, Reliant's
phasers were its only means of attacking Enterprise
that had a reasonable chance of success.