Harmony - Troop Symmetry

As you yourself noted, "BB" is a very strong bonus for attack (there is no equally strong bonus for defense).
And what is your logic? Do you want to further strengthen the "attack" skill? So that "defense" has even less meaning? So that melee units of the player even more likely "kill" the enemy in one hit (i.e., without retaliation)?

My logic is to have interesting skills, not a minimal attack combined with overpowered perks. The same BB could be moved to level 3, which would seriously weaken the attack branch and compensate for increasing the base to 10/20/30%, and Retribution could be moved to Leadership. And if BB and mass acceleration are also separated into different branches, it would be ideal.

In Heroes 3, attack gave 10/20/30%, and no one complained about it being an overpowered skill.

Defense will not lose its meaning in any case, because it is not just a good skill for a warrior (due to significant bonuses), but also almost indispensable for a mage, while attack is generally not needed for a mage (sometimes it makes sense to take it only for the available BB and not upgrade it above level 1).

Another example of a minimal (useless) skill is Counterattack, which does not give any bonuses on its own (much weaker than even attack), it only allows you to train troops, but a cockroach can do that too. Counterattack at +25/50/75/100% could be competitive.

I don't know what you're talking about.
At the beginning of the game, damage is what decides the outcome of battles on the map, it is what allows you to win battles without losses, not defense... or are you not playing on "Hero" difficulty? "Defense" will not give you such a bonus for any castle.
It is "attack" that gives bonuses to Furies, Fairies, etc.
How does "defense" work? Does it prevent the enemy from dealing any damage to your troops?

I'm not interested in theory; take 2 identical situations, differing only in skills, and try to defeat neutral creatures in one case with 30% defense (without perks), and in the other case with 15% attack (without perks), and compare the losses.
Because with such arguments, you can say that even 1/2/3% attack is better than 10/20/30% defense, since there are no practical measurements that would show the real difference.

Real damage output is most often based on ranged attacks or magic (where the hero kills), so defense is needed both there and there to proportionally reduce losses, which cannot be said about attack (we are talking again about basic skills, not perks). This is especially noticeable in really strong guards, which cannot be taken without significant losses by melee units (even with 30% "attack").

I remember even when I was breaking through with orcs (through executioners), I mainly prayed that there would be enough defense and not to lose an extra executioner (every unit of damage counts), and not that I lacked some 15% damage.