onlyme wrote: I guess that is my point as well.
A Thief with 18 dex and studded leather would have a 7-4=3 AC, same a fighter/cavalier with 18 dex in full plate. I definitely understand that someone without armor would be more agile, but I think this rule negates the usefullness of armor. i.e. the plate should absorb the sword or claw more than leather, which is why it has a lower base AC. If you have plate and 18 dex, you should be able to avoid your exposed neck, face, armpit, etc. more so than someone in plate with a 10 dex.
But that's not the same AC, at least not all the time.
It's the same AC if the thief is fighting with one or two opponents, right in front of him, on open ground where he is free to maneuver and take advantage of his high dexterity. But that same thief, when being attacked from behind, or by more than 3 opponents (guestimate here), would just have an AC of 7 [EDIT FOR CLARITY: would have an AC of 7 vs. any of the opponents aside from the initial 3). The cavalier would still have an AC of 3, even if he had eight opponents surrounding him.
(And that is, of course, assuming your DM lets your thief wear studded leather armour... hehe)