This rule is for those that want to spice up their games a bit by manipulating the existing (useless) unit traits.
Leaders and commanders in my opinions shouldn't be treated like any other regular infantry with useful abilities and high command icon access, but the leader and commander traits serve no function other than as a job label. The morale rule below instills the special nature of a leader or a commander in a warzone. The troop penalties reflect the confusion resulting from a superior officer's death...even the near mindless monsters need the firm guidance of a commander or a leader in order to function as an effective battle unit rather than loose cannons.
Leader : When a human LEADER dies all friendly INFANTRY units belonging to his faction reduce their move speed by 1 for the remainder of the round. When a Dark Legion LEADER dies all friendly MONSTER units belonging to his faction reduce their attack accuracy by 1 for the remainder of the round. The above does not stack with the death of a faction Commander (see below). Note that reducing accuracy for monster units effectively gives your monster's targets Dodge +1.
Commander : When a human COMMANDER dies all friendly INFANTRY & LEADER units belonging to his faction reduce their move speed and faction command icon by 1 for the remainder of the round. When a Dark Legion COMMANDER dies all friendly MONSTER & LEADER units belonging to his faction reduce their attack accuracy and faction command icon by 1 for the remainder of the round. Note that Karak is considered to be a Dark Legion commander for purposes of this effect.