Actually a "weapon of mass incapacitation" that had a long-term (though not necessarily permanent) disabling effect, rendering the enemy incapable of fighting, but leaving them still alive and in need of care, would be even more useful than more lethal weapons. The point of war (which the great Prussian military theorist, Clauswitz referred to as a continuation of policy by other means) is to cause the enemy to bend to your will, not to kill them, though killing lots of them has been the usual way to accomplish the bending to the will down the ages. (cf. also Sun Tzu's dicta concerning war and generalship.)
To see an illustration of my point from back to the days when killing prisoners was not regarded as an indefeasibly evil war-crime, Basil II's blinding of the captured Bulgar army was infinitely more effective at dismissing the Bulgar threat to the Empire than killing all the prisoners would have been.