Shakti exists, but so does Galtran, who feels great with armor. Offense is a good help for those whose strike force consists of melee units. But Rampart, for example, needs ranged attacks and defense; attack is certainly needed, but much less.
Added 1 minute later
Perhaps it's due to the tech tree, or perhaps in the early stages, the advantage lies with the one who has Attack, but it would be interesting to know if the heroes have more than one unit in their army. My intuition suggests that the more numerous the army on both sides, the more Defense wins.