1-2 heavy armor units in an army is ok if they can hit hard, trouble is they can never seem to hit as hard as an archer or staff user in the rear on wargs.
I prefer to maximize dodge, accuracy and attack. I use the champion piece that gives accuracy bonus just because it contributes to that, cloak of shadows is great... I can't be wasting my metals and crystals on armor.
I just leather up, and use crystals/metals on accessories or "choice gear".
Edit:
Your right, Dexers make better tanks, I had a guy with dodge really high and he blocked 8 enemy units. I just knew if I traded every point of Dodge for Armor or Defense, he would have been dead.