Marshall said:
There is also the point that the difference between Light and Heavy Armor is 4 points. At best thats a 20% advantage that falls fairly quickly to irrelevant. The penalties to heavy armor quickly outweigh any benefit, from a measly +4 AC.
This is actually quite innaccurate. +4 AC may seem like a very small advantage, but it is often very significant in regards to how often you're hit.
Consider the following characters:
Aggressor: Human Ftr 2: str 16, wp focus longsword, masterwork longsword. Total attack bonus=+7
Lightarmor: 14 dex, chain shirt, large shield, dodge feat. AC=19
Heavyarmor: 12 dex, fullplate, large shield, dodge feat AC=22
In order to hit lightarmor, aggressor needs to roll a 12.
In order to hit heavyarmor, aggressor needs to roll a 15.
Consequently, aggressor will hit lightarmor 45% of the time.
He will only hit heavyarmor 30% of the time.
So a +3 differnence in armor class translates into a 33% decrease in damage taken (all other things being equal) which is much better than a 20% difference.
This effect is much more noticable when armor class is better compared to the attack.
For instance, consider lightarmor and heavyarmor fighting goblin rogues (lvl 2, str 12, wp focus shortsword. Total attack bonus=+3)
The goblin needs to roll a 16 to hit lightarmor.
The goblin needs to roll a 19 to hit heavyarmor.
Consequently lightarmor will be damaged on 25% of the goblin's swings but heavy armor will only be damaged on 10% of the goblin's swings.
In this case, the 3 point difference in armor class translates into heavyarmor avoiding 60% of the damage lightarmor takes.
Now, it's certainly possible to max out armor class without wearing heavy armor. On the other hand, for characters without a super high dexterity, it's a lot cheaper to buy fullplate and boots of striding and springing than it is to buy a +4 chainshirt (or a +3 mithril breastplate).
The difference between a 31 AC (+5 fullplate, +5 shield, +1 dex) and a 28 AC (+5 chain shirt, +5 shield, +2 dex) is also very significant when facing creatures with +13 to their primary attacks (Annis Hags for instance). (It actually about a 50% difference in how often the characters will be hit).