>>314854245But that other poster was right, now that I think about it.
Think about it, anon. Let's say that you're wearing full plate, and you tank a blow from a pollaxe in the chest, or deflect it with your arm. Even if the armor does it's job at preventing your bones from being shattered, it's going to hurt like a motherfucker. It's probably also going to throw off your balance and/or knock the wind out of you.
By blocking with a shield, you retain far more control of the situation, by being able to redirect the strike and send your opponent off balance, etc. Not to mention the (relative) lack of pain and suffering upon getting hit. You can also use it as a weapon by bashing with either the face or edge of the thing.
Shields have more utility than just keeping you alive, I can't imagine they were dropped entirely even by warriors wearing full suits of plate