Heavy armour is important because not everyone can afford high dexterity. Full plate can grant you nine AC from armor plus max dex, whereas breastplate (the best medium armour) and chain shirt (the best light armour) give you only eight.
Essentially, the best two armours in the game are chain shirt and full plate. Full plate with twelve Dexterity will give you one more AC than chain shirt does on 18 Dexterity; chain shirt, however, has no speed penalty.
If you want to sneak, then the Armor Check penalty is significant, so rogues (who work best on high Dexterity) should go for masterwork studded leather (no penalty) or masterwork chain shirt (-1 penalty, but you get an extra point of AC without needing 20 Dexterity).
If you want two-weapon fighting you have 15 Dexterity anyway, so you can afford to build a character reliant to some extent on Dex. Of course, here’s the thing: if you wear breastplate your AC benefit caps out at +8, and since you’re taking the speed penalty you might as well go for full plate (AC+9 overall).
If your Dex is 18 or higher, you can afford to wear any of the light armours. There’s largely no reason to wear anything but chain shirt unless you care about the Armor Check penalty or the cost at first level, unless you’re in the rare position where you have 26 Dexterity or more and can have +9 AC using padded. However, by that stage in the game you can buy Celestial Armour or somesuch light stuff.
Basically, if you’re anything above Light armour you might as well go for full plate. If you’re wearing light, you may as well go for chain shirt, and if you have 26 Dexterity or more you should wear Celestial Armour. Only if you really care about Armour Check penalties or find something special in treasure should you bother to wear any other armours.
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