Rolling the dice to decide on the accuracy of a gunner to hit his target makes a certain amount of sense, so a "0" point hit could exist.
I'm not sure there will be gunners in the true sense in space battles.
But I cannot see the logic of rolling the dice to determine the effective strenght of the shields.
Direct hits vs glancing blows - point defense actually makes sense. Not saying the system is good. But it treats all weapons and all defenses in the exact same way, so you can't really expect too much from it. I'd prefer a more realistic system, with different weapon types having different effects. More strategy in actual combat, not just r/p/s "strategy" in ship building.
A shield having "X" power should deflect all hits of "X" or lesser attack levels.
I once suggested something along those lines. But it takes a complete overhaul of the system.
Your efficiency idea, I don't think it makes too much sense.
Therfore, a "Shield" with a strenght of "2" and an effeciency of 75% would now be superior to installing two "basic deflectors".
It's all about cost and size.
For example, deflectors could have an effeciency of 50% against lasers but only 40% againts phasors or disruptors!
Though something like that would be awesome, the way the system works it's really of little consequence.