Basically Blade Shield is like a weapon that swings every second at every target and has a range of 1, because typically you can dmaage a monster simply by running by them.
That's not all it does, and your right it is overrated. Blade Shield, unlike Blade Fury, will trigger Life Tap on chance to cast items (Last Wish, Dracul's Gasp)
Then, as an added bonus, it heals you when Blade Shield hits an enemy while they are tap'd. You can be mobbed by more than 20 non-undead monsters (and yes, I HAVE tried this and it is true) and they heal you as they hit you, it is fantastic. Eventually, because I had a Last Wish, maxed out venom and Drac's on, the mob just kept hitting me until each fell to the ground.
Also since Blade Shield depends on weapon damage and I had Last Wish (Might aura) and my Rogue merc had Faith bow (Fanatacism,) the damage from my Blades was very strong.