It does seem that the HP of shields look fairly similar to RQ3 APs.
So, a Target Shield has 12 HP, a Kite Shield has 18 HP.
So, using the HPs to parry with would be consistent with RQ3 parrying values. Then the APs would be used to protect weapons when the weapons are being attacked.
I've just noticed somethng when reading the SRD.
Strike Weapon / Shield
The attacker declares that he is attempting to attack the target’s held weapon or shield. If the attack is successful, any damage is dealt directly to the opponent’s weapon, using the rules for attacking inanimate objects. Natural weaponry may not be attacked in this way.
and
Using a weapon on an inanimate object with armour points equal to or greater than that of the weapon deals damage on both the object and the weapon.
So, since both target and kite shields have more APs and HPs than any weapon on the table, attacking to damage a shield means that you are more likely to break your own weapon than the shield.
An example. I have a 1D4 Damage Bonus and a Bastard Sword which does 1D8 damage and has 4AP/12HP. A Target Shield has 8AP/12HP. I can't see how you can break the shield first.
Attack 1: I do 10 points of damage, 8 get blocked by the shield and the shield takes 2HP damage, but my sword takes 10 damage, 4 is blocked and the sword takes 6HP damage. Shield 10HP, Sword 6HP.
Attack 2: I do 6 points of damage, the shield blocks it all, but my sword takes a 6 point blow and takes 2HP. Shield 10HP, Sword 4HP.
Attack 3: I get lucky and do 12 damage, the shield takes 4HP damage and my sword takes 8HP damage. Sword breaks and shield hangs there looking smug. Shield 4HP, Sword -4HP.
OK, perhaps the damage done to the shield is revisitied on the sword.
Attack 1: I do 10 points of damage, 8 get blocked by the shield and the shield takes 2HP damage, my sword takes a blow of 2 but the armour blocks it. Shield 10HP, Sword 12HP.
Attack 2: I do 6 points of damage, the shield blocks it all, nothing comes back on the sword. Shield 10HP, Sword 12HP.
Attack 3: I get lucky and do 12 damage, the shield takes 4HP damage and my sword blocks 4 taking no damage. Shield 4HP, Sword 12HP.
Attack 4: I do 11 damage, the shield blocks 8 and takes 3HP, but 3HP bounces off the sword. Shield 1HP, Sword 12HP.
Atack 5: I do 12 damage, 4 gets through, shield breaks and sword is unharmed.
In this situation, the sword cannot possibly take damage.
Which, if any, is preferable?