My interpretation has always been that while most nobles participate in combat, they are either surrounded and protected by their unit, or at its back. Thus, they only personally engage the enemy when the unit is fully embroiled in a melee.
Heroes, however, fight at the front of their unit, rallying them forward with their own courage—and potentially paying a high price for it.
Heroes don't really fight at the front though. Also the price they pay is probably not that worth it when there is a better subclass that won't get your character killed.
