Take Tuatha as an example. A Troll champion with +6 health regeneration per turn. In addition to being a defender (+1 hitpoint per level) he also has the Tough trait (+3 hitpoints per level). Tuatha has 0 defense, and his only ways to boost that are spells, shields and maybe some accessories.
As a melee/tank champion, Tuatha gets flattened in battle. His defense is extremely poor, and his hitpoints are nowhere near enough what is necessary to survive against even basic militia.
There are plenty of other heroes that have to give up significant capabilities for defense, trading them away for seemingly weak talents. While it makes sense from a purely aesthetic perspective, I really don't see it working out balance-wise. The lack of defense only serves to expose how poor AI is at choosing weak targets. Meanwhile, if I face them in battle, they get destroyed instantly as lack of defense = death.
Perhaps something simple as giving them +1 defense per level would help.
I agree with your claim, but... till the changes be made you can:
1. give cain belt +4 def
2. Give him kite shield or similar +6...8 def and 15 def if defending (and thats the key of his role).
3. Give +3 def +3 magic rez ring
4. Gvie regenerating dager +3 hp per hit.
5. Give turtle +3 def
6. Pick defending, endurance traits.
7. Pick healer and guard skills (guard gives defence not only to guarded units but to him also.
8. give initiative rings, bags, stones, weapons.
What it gives:
ussage of his regeneration as more often as posible, keeping in defending raises his defence to 50 or more, dagers gives hime even more regeneration as he can counteratack while in defending phase. He can use first aid if geting low. You can also give him snacks and other stuff.
As a rezult he can outstand tons of damage while doing none 
Give hime one heavy armored stack and other may be ranged units and hes ready to lead his own army.
Its not hero, its you, who can find out best usage of everything in this game and thats why i like it, you have tons of options: use them. Maybe hes not your general in front of lines, but he can serve well in protection of your other side borders.