You could say the same about burns. Your fallacy, though is assuming that all of the modifiers equally increase the difficulty. In reality, the majority of the difficulty is given by the epic or nightfall modifiers, which significantly increase the damage needed to kill an enemy. Burns give the players increased damage, but they balance that "benefit" with the ability of enemies to one hit kill you. Applying light switch to player melee attacks would not significantly reduce the difficulty because it would still be very difficult to kill an enemy with melee and survive.