I'm a bit late to this discussion but armor in melee and ranged combat is pretty complicated.
The light armors (Cloth, padded, leather) are simply meant to cushion blows or turn near hits. The user should primarily use his weapon or his movement to get out of the way. A fight between people without serious armor is over in 1-3 exchanges: 3 when you both suck, 2 when you are equal and 1 when either is better.
Medium armors, such as cuir bouille, scale and maille can stop a lot of damage but there is some way of circumventing the defense so the user still needs to do a lot of fencing or the armor protection will be worthless. Maille is pretty bad against Krush, Scale and cuir bouille can be penetrated by Puncture easily. Medium armors do work against slash or hlancing blows decently. So a fighter with medium armor is safe from most glancing blows.
Heavy armor (Lamellar and Plate) is designed to work well against most attacks. In a fight, the emphasis lies on circumventing the armor, not penetrating it. This makes a fight very one sided if an armor user is fighting someone in light armor.
A fact that I discovered wearing armor personally is that defending yourself as if you are wearing no armor in a full armor is going to make you fatigued very quickly. Better to concentrate on getting your opponent down than to avoid getting hit. This is not to say that you should try to head bash every attack from the opponent, but you will be better off trying to minimize your defense in favor of offense.
Shields play a big role in employing the right defense. 2 handed weapons never were common before the advent of full heavy armor. A shield was integral to a person's defense, either in a shield wall or when fighting in a skirmish. Also against arrows a shield was the main defense.
Helmets have been indispensible since forever. A shield and helmet made a warrior at least somewhat safe.
Also one should remember that armor was made to stand against the weapons of the day. Celtic and Roman mail were very different from the maille of Charlemagne or William the conquerer and those were not the same as the maille worn by 16th century men-at-arms. Cuir bouille was very prominent in the far east but nearly non existent in Europe. Europeans liked stabbing a lot and Eastern warriors took more slashing and hacking. In the Americas armor was pretty undeveloped. Against their weapons (wooden swords, clubs, spears, etc) the best armor was air cured leather with fur attached. So that was what the conquistadores encountered, while they wore plate.