There are PnP systems where armor primarily functions off of DR, and platemail and the like accordingly become king per realism. D&D 3.X is not one of those systems - it is balanced around AC being the primary representative of your physical defenses. When you fail to get hit, it's not just a blow missing you, it could be construed as glancing off your armor, your shield, hitting a tough portion and failing to make through, etc. Changes to the armor system to incorporate DR would be a half-measure step away from 3.X without accommodating all of the considerations those other systems make for their armors.
Let me throw you some numbers.
A tower shield provides 3 AC. Base plate armor provides 8 AC and allows for 1 Dexterity bonus (let's say you have 12 Dex or chug a Cat's Grace potion, either common for frontliners).
To match even that 12 AC, without assuming that the shield or armor has been buffed or is of good quality, you need to have a combination of armor, dexterity, and parry that reaches it. I'm not counting tumble, as even a fighter with a minimum dip into rogue can match a level 20 rogue's tumble and get full benefit from it.
All armor on this server has been balanced to meet 9 AC between Base Armor Class and Maximum Dexterity Bonus. A rogue wearing Studded Leather would require 24 Dex to meet a 12 Dex Fighter in Full Plate's AC, and they'll also require 15 points of Parry to cover the tower shield's 3 AC. That's fully half of their total possible parry devoted simply to catching up - and it's very simply done to make a tower shield +3, if only temporarily, which would even it out with the full 30 points.
As to setting integrity - this server is already balanced so most content is difficult to reliably accomplish without being lit up like a Christmas tree. Nimblemen having higher potential AC at the levels where people start developing almost super heroic talent doesn't bother me too much, particularly considering the prevalence of enemies with high DR and immunity to the classic dex methods of damage pumping hardly render strength's extra damage meaningless.