DB should only be used when you have no armor or shield bonus, and shouldn't be a better bonus than an average character of the same lvl wearing armor. That would reflect the character using his fighting prowess both armored and unarmored but the armor adding some protection.
If Iwere to keep it simple I would add a DB spot to table 3-1 and give a character the bonus as he lvls like BAB and saves and just add an unarmored spot to the character sheet.
There are a couple of game balance problems that immediately pop to mind:
1. Classes that don't have armor proficiencies gain plenty, whereas those who do have armor proficiencies gain nothing at low and mid levels. The monk for example, now has his normal AC bonuses PLUS his BAB. The arcane classes also gain since they never wear armor. The rogues who do not wear medium or heavy armor because it restricts most of their class skills gain as well. The "tanks" will only really gain once their DBs are higher than the AC bonus provided by the armor and shield - REALLY high levels only. The way you're suggesting does not have equal/fair benefit to the armored classes.
2. Even at really high levels there is an anomaly. It seems a little strange that the armored class, who have been fighting in armor all of their lives, will one day say: "Hey, who needs this metal junk, I'm much better off without it!" The fact that some classes have better armor proficiencies than others, is supposed to give them an advantage throughout their careers. Especially as they have been advancing in their martial prowess with armor all of their lives.
3. Logically, it does not make sense that the DB should not come into play even while wearing armor. The way we understand it, it is a parrying skill. A fighter who is wearing armor, even heavy-heavy armor, is still going to try to the best of his ability to dodge, parry and block. He doesn’t just stand there and say “OK. Give me your best shot buster.”
Sometimes keeping things too simple is not the solution. My suggestion is really not so complicated. Let me sum it up:
AC = 10 + Dex + Size + Shield + BAB + ACP
Touch = 10 + Dex + Size + BAB + ACP
Flat Footed = 10 + Size + Shield + ACP
DR = AC bonus of armor.
This way, all classes benefit proportionally and fairly, at each level.