There are several reasons why Shield and therefore Shield Rating isn't that useful for PVP.
Shield is calculated after Defense. I.e. avoidance is checked first, then mitigation.
As I wrote in the Guide:
- Shield applies to weapon damage
- Shield does not apply to Kinetic damage
- Shield does not apply to Elemental damage
My guess is that Shield does not apply to Energy or Internal damage either.
Which basically means that Shield only mitigates a limited number of attack abilities.
Weapon damage is either Kinetic or Energy. Pretty sure it applies to both.
As to Elemental and Internal, I'm not sure. It makes sense for it not to apply at all to Internal and I can buy that.
Don't get me wrong, I'm not a fan of Shields either, I think it's a clunky mechanic that requires a LOT of investment for little returns. I'm just saying that you probably have what it does and does not mitigate incorrect.
Edit: I'll find someone in guild to duel to fiddle with this a bit, see what I come up with.