Damage calculation in battle.

it has long been proven that defense in the third game is significantly more effective than attack

To be precise, the Attack and Defense skills are roughly equal and have advantages depending on the situation. However, if you compare specialists (say level 15-20 and beyond) in Attack and Defense, assuming both have Expert Attack and Expert Defense (proper builds), the Defense specialist turns out to be stronger.
If we're talking about a hero's attack and defense stats, an advantage of a few points in attack often proves more effective than in defense. But as the gap grows, defense specifically takes over; that's why without magic, a top-tier swamp dweller is significantly stronger than a top-tier Barbarian. That's why Tankozavr is Tankozavr (do you know how dragon tooth fragments are obtained? Exactly: they send Tazar with Expert Earth into the Dragon Utopia, and the dragons break their own teeth) + the swamp has many sturdy units.

because taking only 10% damage is imba

Well, you're exaggerating a bit. For that to happen, Tazar would need to be level 50:
0.3(defense cap)*0.7(Expert Shield)*(1-0.15-50*0.05*0.15)(Defense+spec)=0.1
At a typical level 20, you get approximately 15% damage, but only if your unit's defense is higher than the enemy's attack by 28 — which only happens against neutrals.