[MaroonMaurader]

If you decide to add hero defense to unit defense, you pretty much have to do damage as a difference rather than ratio. Suppose Amelie has 25 defense. Then if things are done as a ratio, she cuts damage to a Black Dragon stack in her army by 26%, but cuts damage to a peasant stack in her army by an astonishing 96%.* Now suppose you balance peasants and black dragons so roughly equivalent leadership, on their own, are roughly equally dangerous. This means that when a 25-defense Amelie has roughly equal leadership of them in her army, the peasants are *vastly* more dangerous than the black dragons, because they're practically invincible compared to the dragons.
On the other hand, if the damage is done by difference, then for both the Black Dragons and the Peasants Amelie is changing the difference between attacking unit's Attack and the defender's Defense by the same amount, and thus changing damage by roughly the same factor - so units that are balanced to work on their own remain balanced when working with a hero boosting their attack and defense.

*A quick example in case it wasn't clear where those numbers came from...
If damage is done as a ratio, then the damage formula is something like...
Damage = BaseDamage * (Attack/Defense).
Damage (with Hero defending) = BaseDamage * (Attack / (Defense + Hero Defense)).
The ratio of the two, Damage (with Hero) / Damage, is then...
Base Damage * (Attack/ (Defense + Hero Defense)) / [BaseDamage * (Attack / Defense)]
This simplifies out, giving...
Damage (with Hero) / Damage = Defense / (Defense + Hero Defense).
When Defense is 1 and Hero Defense is 25, that ratio is 0.04. When Defense is 70 and Hero Defense is 25, that ratio is 0.74.