There is some inconsistency to the mechanics of shields.
Some provide Cover v ranged attacks, some increase Armour Points on the wielding hand and body (protecting vs both ranged and melee attacks even if you fail your parry), and some (like the Arbites suppression shield) do both (though I can't see the AP value of the cover it is said to provide?)
I can see why only large shields should be used for cover, but why should the Guard and Naval shield not provide APs like the Combat and Suppression shields do? (Granted, the combat shield is a Rogue Trader shield not included in your fabulous handbook, but the point stands. In Only War, all shields provide +AP to body and the wielding arm).
Per RAW, you cannot take Cover from melee attacks, only from Ranged attacks. But if the "Cover" in question is strapped to your arm, and you're holding it in front of your opponent…
Then there is the issue of effects from craftsmanship. Shields are weapons, not armour. But their purpose is to reduce/avoid damage, not to attack. The DH rules specify that the WS Bonus from craftsmanship is just for attacks, not parries. Later games changed this to apply to all WS tests. So that improves their parry usefulness, but APs do not change like with armour.
If you were to unify the mechanics for all the shield-rules, how would you handle it?