Just come out now and admit that it doesn't make any sense.
Just because you haven't heard of it doesn't make it wrong. A chain shirt doesn't protect the whole body, or even the whole torso against some weapons. Leather bracers don't protect the whole body either. Even when wearing chain or leather armour all over, there are usually gaps where these parts overlap which can be exploited. 'chain armour' wasn't usually just chain all over. Traditionally it consisted of padded armour made from tough cloths with chain placed over the top. Weak points would probably be in the head, neck and waist, depending on the style of the armour.
You have obviously never worn leather or chain armour, or probably any armour so you have no idea what you're talking about for the rest

Moving in chain is different to moving in leather, you are restricted in different areas and they have different weights. You can be the strongest and quickest man around and be a good fighter, but if you've never worn armour before and someone puts you in a suit of chain and tells you to go fight a guy with a mace, you'll die in 10 seconds. As I said at the end, which you ignored, being able to move around as if the armour is just skin takes some time to get used to.
Number two is we are fighting MONSTERS here most of the time. Not master swordsmen who are actually aiming for the weakpoints.
A monster might not be looking to hit you in a certain area, but it might get a lucky swing.

Being able to position yourself correctly and be ready for anything heading in that direction is important. Doing that naturally without thinking takes time. Against opponents with different weapons you also need to be able to 'naturally' know how to protect yourself best. Chain is pretty useless against crushing weapons unless you know what you're doing, and great against slashing especially if you know the correct way to take the hit.
I do think that the 'learning from a npc, then practising' doesn't really make sense for armour, and that it doesn't really make sense for quite a few skills. It's something we've had on the todo for some months to change to be better. We certainally won't get rid of the armours as skills altogether though.