Now, after a lot of play, I started to suspect the exact combat mechanics of "parry". Please someone to confirm if this is correct.
1. So it appears the shield actually is *absolutely* useless if you have only 1 enemy (like the arena battles). It is said the shield adds a second parry but it seems the shield works *only* for the second parry but not for the main parry. Which suggests all the shield related skills are useless if you have only 1 enemy.
2. Which suggests in all the battles against only 1 enemy - including against mini-bosses (Zant etc) the shields are actually harmful because they reduce the attack w/o helping the defense. And even vs more then one enemy it maybe is still better to use a shield with less attack penalty and less parry value instead of more penalty and more parry value.
3. On the same time the defensive combat skills seems to work *only* with the weapons parry - which means only for the first parry (not sure about this - needs testing)
4. And it is not clear but seems logical the weapon parry modifier to work only on the first (non-shield) parry. Still this needs a confirmation (easy be tested too)