If I'm not mistaken metal plates will actually be fairly uneffective when used against bludgeoning weapons. I think it'd dent in when hit, as it normally is quite a bit larger than the person within it, allowing open space, which would cause severe trauma to the affected body part.
First off, I'm dumb for saying that you were wrong about
quite a few things. Looking back at what you said, you were only wrong about one thing, which was saying that plate armour was ineffective against blunt force weapons. Also, sorry about mispelling your name. :3 And also, I had a hard time finding good sources to link to, but a lot of what I will be saying in this post is from information garnered from reputable individuals who study this and make armor for a living.
Plate could and would dent against bludgeoning weapons which is part of why hammers, picks, and maces were frequently used against them. The other reason is that most other kinds of weapons couldn't as easily damage someone through plate armour, as plate was able to easily deflect these weapons. (
http://en.wikipedia.org/wiki/Plate_armour)
But, this did not mean that plate armour was ineffective against bludgeoning weapons or less effective than other armours against these weapons. Plates still could deflect these weapons, though not as easily, and will disperse the force of a strike over a broader area than if there was no plate at all. The solidity of plate is what allows it to do this, along with some presumed padding underneath it. The rigidity of the plate allows an impacting force to move the plate in its entirety, and the plate being broader than the point of the weapon impacting it, will impact the underlying padding and body under it with a much broader area. This disperses the impacting force, hopefully reducing the likelihood of permanent and life-threatening injury. If the plate does dent, well, it'll suck for the wearer of the armour, but if there's some space between the plate and the body, it might not be deep enough to be problematic and the impacting force will still be distributed over a large area. If it is problematic, then you would have had problems even if wearing any other kind of armour. Plate armour was effective, just not invulnerable to such weapons. And plate does a better job of protecting its wearer than any other medieval armour against any medieval weapon.
Keep in mind also (and Caesar was right about this) that helms, plate, and maille armour invariably had padding under them which was absolutely vital to the ability of these armours to offer protection. Maille was worn with thick padded garments under it. Check out some of the links in my previous post to see what I mean about maille and the padding underneath it. Plate was frequently worn with maille under it as well as padded garments under that.
Helmets weren't just metal sitting on the head, but often had padding situated as so to suspend the helm over the head. Here's a good article on the matter:
http://www.highlandcitadel.com/fencing/HelmsWeb.pdf A correctly made helm (and I have first hand experience with this) will not ring like a bell when hit. A poorly made one will, and it sucks. Getting hit in the head, helm or no, sucks and is disorienting at best. But a helm increases the odds of surviving a strike to the head big time, and plate head defenses, when correctly suspended and padded will disperse a considerable amount of force from a blow, even one from a blunt weapon or a rock being thrown at your head.