The higher the block on the shield, the more valuable each point of Shield Specialist is.
For example, let's say I have 20% blocking. This means I have 25% increased survival against attacks (5/4); for every five hits coming at me, I eat four of them and don't take the fifth. With +2 Shield Specialist, this goes up to 30% blocking, which is 42% increased survival compared to 0% block (10/7), which is 14% more survivability than 20% blocking (10/7*4/5).
Now let's lock at 50% blocking. This is 100% increased survival against attacks compared to base (2/1). However, +2 Shield Specialist takes me to 60% blocking, which is 150% increased survival against attacks compared to base (5/2), and 25% more survivability than 50% blocking (5/2*1/2).
This means that Shield Specialist has far less diminishing returns than most skills, because the higher the block chance, the higher the survivability increase. If you can go from 75% block to 80%, that difference is absolutely huge.
