TO put the emphasis on some point developped by Markc
A shield bash is made with the rim of the shield, or the boss, or a point. If you just slam the shield on your foe you won't wound him, for sure.
A human fist can break a bone and kill instantly if the strike is well aimed, a shield just has the same effect, with more mass and more resilience.
Besides this, if you compare a ram headbutt while charging, compare it to a shield bash while charging. From what i've seen, I don't want, ever, to receive a shield bash from somene who was charging me in the head, throat or belly. I love my life and I expect it to be long and fruitfull
PS : results from 10 minutes of fight with blunt shields and swords (the shields were light metal, the rim was not sharpened) and no armour at all.
Me : Forearmes badly bruised, my own shield managed somehow to slam my left arm and I ended up with a blue wrist. The sword itself left me 5 red marks on the chest.
Partner : skin bruised and cut in the elbow pit from a shield bash. Sword strike in the belly left him a nice, red horizontal mark from one side to another, another shield bash punched him in the face (I held it back when i felt the contact, mind you) and the left side of his face turned red and blue quite quickly.
Why do we fight unarmored, at our own risks ? Because even with blunted, sparing weapons, we can send our opponent to the hospital in pretty bad shape. We ned it to fight with a sense of danger, know our distane, measre the strength inthe blows. This way, we can learn to "feel the steel" ("sentir le fer") which is a very important part of armed combat, and gain what in RMSS is called the "combat awareness". We do it under control of arms masters.