I think this is incorrect reasoning. Let's assume hypothetically (since Dwarves have increasing defense, not attack) that a unit has 100 defense and 50 attack.
Which is better: increase defense to 130 or attack to 57.5? Tactics are irrelevant if you have the Energy Rune. And with defense, you have the "fight to the last" ability, which is simply amazing with the Resurrection Rune. Also, a defensive hero with Steadfastness turns Warriors and Spearmen into an invincible mass.