For starters, every innocent victim of the psychopath and/or terrorist in Aurora, Colorado. He wore armor. The same people could have used sound suppressors, both to avoid being blinded by their own muzzle flashes in the dark and to reduce recoil.
In the absence of vest-busting ammunition, any pistol with any rounds would have been better than none. Arms, head and legs all have major blood vessels. Arms in particular are often hit when armed suspects are shot — no instant stops that way but a major impediment to further slaughter of innocents.
Those who have been to firing ranges know how much concentration is required to make accurate hits. Return fire, even if technically ineffective, still has a way of breaking the attacker’s concentration. And most trained people would have eventually scored hits. Had more than one lawful gun carrier been present, Mr.Psycho would have ended up in a crossfire — again, not a position conducive to marksmanship against moving people in semi-darkness. But he got to shoot helpless, unarmed marks without any fear of return fire — in part thanks to the theater policy prohibiting lawful carry, in part due to the complacency of the theater-goers.