But on the flip side, Spellcasters will never have much AC anyway, and at high levels a +2 natural armor bonus won't matter a bit against the +34/29/24/19 Fighter, while the extra 20 hp (on average) will mean something.
Also, giving one class a base +2 to AC will mean the powergaming munchkins take one level for the AC bonus and go on their merry way. The closest thing is the Monk's Wisdom bonus, and they lose that when they wear armor.