I think armour is over rated. If you get hit while wearing armour you almost always feel the blow. Sometimes there partial penetration of the armour. Armour doesn't prevent you from being hit, it prevents you from being killed. Or of dying from from your wounds. Take chainmail for example. It didn't stop any of the energy of a sword blow. It stopped the blade from cutting you. In the middle ages infection was the deadliest killer. You came away form a fight with brusing and maybe broken bones (which they did know how to heal). Not dying from bleeding out or getting some nasty rot in your wound. Armour was continually improved over time. But improvements in armour were matched with improved weapons. In fact the origin and popularity of some weapons can be traced to a specific armour type worn. Armour was an insurance policy against being dead. It was your last line of defence.
Now that I have hopefully shaken your faith in armour let me build your confidence in shields. Look at the shield. Its made of thicker stronger materials. It stands out away from you body with only your arm conecting it to you. By the way your arm is a good shock absorber. It covers your body and hides you form you enemies, they can see the shield but not you. Its portable cover. In melee comabat to avoid being hit you have to dodge. That requires room to manuver. With a shield you block. Without moving, stationary in place.
And now to put into game terms (for D&D at least). The no amour AC rating I think assumes room to move and dodge(remember the -4 AC penalty for being prone). The sheild only AC while being only 1 better than no armour doesn't require you to dodge. So if you are not wearing armour and not dodging your AC should be 13 (original scale) without a sheild and 8 with. Thats + 5 bonus. However the orignal AC chart shows leather alone to be one better than just a shield. So again the shield gets the shaft (pun alert). Why? Maybe because the shield is not something you wear. Its not custom made for you or fitted to your body. And it can be tossed away when damaged. So its temporary not like the more constant armour. Not very heroic. So whats wrong? Fighting with a shield is not the same as without so the AC chart has an apples and oranges problem. Another problem is that while the chart has a number of armour types there is only a single bonus for a shield. So all types of shields are lumped into one.
So now we have a bunch of armour types and only one shield. What if I reversed that. How about a number of shield types and one armour type. Oh no, cry the armour people. Plate armour protects better than chainmail, which is better than leather. I disagree. I would argue that better armour is more durable. Wthout going into details of weapon type vs armour type ala AD&D suffice to say armour and weapons keep pace with one another. So improved materials make the armour last longer. Taking a page from Mazes and Minotaurs where each piece of armour is separate I have an idea on how I want to do my Shield Class (SC) chart.
I picked three generic types of shields plus a bonus for helmet and armour. Using the original decending AC style. Here is my idea.
large shield ; hoplon or aspis or scutium -SC 4
medium shield; viking round or knight's heater -SC 6
small shield; strap on or bluckler -SC 8
bonus to SC for wearing armour -1
bonus to SC for wearing helmet -1
So a fighter in helmet and armour with a large shield would be SC 2 or AC 2 if you prefer.
Fighting without a shield means you will get hit alot more. But thats what hit points are for.