So one detail mentioned in the WN but not explicitly stated here is that
The armor that Bard smashed in? It was supposed to be some kind of specially tough armor and the helmet was supposed to block the impact/deflect blows to the head.
So Bard being able to cave it in like that implies the helmet would be useless in term of actually saving the wearer's life, in fact it might even hurt the morale even more seeing how he just fucking CRUSHED a metal helmet with a sword.