...And think of all the research that could be done by blowing up stuff with photon torpedoes! Spectroscopists would be elated.
Even if the Grissom
has no weapons (none are evident on the model, after all, and none are specified in dialogue relating to this class even in later episodes), there might be scientific instrumentation onboard that uses antimatter or other volatile materials or devices. A distribution system similar to that which supposedly takes antimatter from the power systems to the torpedoes in warships might exist on the Grissom
as well, now serving exotic scientific instrumentation on the various decks (and the plot need for a dramatic vulnerability).
But having Kruge's gunner hit just the main power system is good enough an explanation IMHO. And never mind that the hit is to the lower part of the vessel, even if the warp nacelles are up high: if we have any "official" information on where the antimatter reactor on such a ship might be, it is the cutaway diagram from "Hero Worship" which seems to place the warp core right where the Klingons hit the Grissom