This will depend to a great extent on the availability of magic items and the orientation of your players (defensive or offensive).
If your campaign is considerably lower magic than the DMG standard world where most 6th level NPC fighters have magic fullplate, this will probably not effect the balance too much. On the other hand, if your campaign follows DMG standard treasure, you can probably expect to see characters like the following ones at 12th level:
Pal 1/Clr 11
Dex 10, +3 fullplate, +2 large shield, Amulet of Natural Armor +2, Persistent Shield of Faith: AC 34
Ftr 1/Wiz 7/Spellsword4
Dex 14 (18 cat's grace), +3 mithril breastplate, Persistent Shield Spell, Persistent protection from evil spell, amulet of natural armor +1, +2 animated large shield: AC 38
And that's before Expertise, Dodge feats, Haste spells, cloaks of displacement, etc.
Under core rules, it's pretty easy to use magical heavy and medium armors to create characters who are very hard to hit. Adding additional armor bonusses to the mix would encourage this further. Whether or not that's a good thing depends upon how defensively minded your players already are. . . .