Why wasn't it just "yes, commander Riker, the Borg are indeed "asleep," the ship is basically powerless, all systems including shields are down."
Riker: "excellent. Fire all weapons."
"the Borg ship has been destroyed."
It's also worth noting that even powered down, the Borg cube had an absolute crap-load of superstructure. The E-D could conceivably have exhausted its compliment of photon torpedoes and still not have completely destroyed the cube completely.