Game Description: Reduces damage taken when blocking an attack with a shield.
Shield Def. Bonus only decreases damage taken when blocking. It does not increase block rate. However, if you feel the need to test this still, grab the shield and hop on DRK.
I fished up 3 crabs and the second one dropped the rusty shield. I had my friend help me create gleaming shield and once it was created, I tried to see if you could etch a slot for evolith but as of now, there's nothing available for it. The lack of extra stats and things to do with the shield leads me to believe the statements about significantly higher block rate and obviously the higher defense will help, although I'm still only PLD78 so I have not been able to test it out for myself.
With the current highest DEF rating of any shield (meaning it has the highest damage mitigation of any shield), and still being a Kite-type (Type 3) shield, this replaces just about every other shield for straight-up tanking, including the Seigneur Shield.
However, in terms of fights that are kited, where you're not engaged with your sword, you'll still want to refer to the Wivre Shield/+1 for the Enmity bonus.