Fifth Element said:
Mearls (I believe) already dicussed the mechanical reasons for not making the change. But if it hurts your immersion, just change how you think about it. Armour does make you harder to hit. If your opponent lands a blow that would have hit you if you had no armour on, the opponent simply hits your armour instead of you. This is particularly easy to think of when wearing plate armour. The opponent lands a blow, but your armour simply deflects it. If not for your armour, he would have hit you.
Prepare for stream of consciousness!
It works like this in my imagination:
Suppose a fighter has an AC of 21.
That's made up of,
+8, plate armor
+2, heavy shield
+1 dexterity bonus
If his opponent rolls a 9 or below, he totally missed.
On a 10 to an 17, he hit the fighter's armor but didn't penetrate.
On an 18 or 19, the fighter blocked with his shield.
On a 20, the fighter ducked out of the way.
On 21+, the fighter got hit.
If I were a truly, truly great DM, I would have a little chart next to each character's AC in my notes, so that I could describe combat appropriately for each person. I really, really should do that for my next campaign. If 4e avoids too many temporary AC bonuses, it won't be so hard. In 3e it wasn't plausible because my PCs were too into buffing, and I'd have had to move all the numbers constantly.
The more I think about this, the more I think I should do it.
...I should probably reverse the order though. An attack that you completely dodge is a weak one. An attack that you block with a shield is a tougher one. An attack that you can't dodge, and can't block with your shield, but which is deflected by your armor is a pretty good attack. So I'll reverse the order, and my players can know from the description how close things were.