First it grants you access to expose weakness, secondly saves aren't that hard to get high enough with items. 19 is enough for most things, evasion is valuable.1. You realize Evasion is only good with a high reflex save right?
What's wrong with Barbarian AC ? Nothing, especially not when raging.
They get +2 AC at level 20 and otherwise if we compare a barb 30 to your http://nwn2db.com/build/?256647 fighter 30 build where the only AC feat you pick are LoH and Weapon supremacy. Picking LoH is fine on the Barbarian so we're left with +1 AC for the babr .. which is offset by the use of a towershield for the fighter so a a small +1 AC for the fighter.
However that's without taking into account the bonuses to AC when raging, which are considerable : between Ice Troll berserker and epic rage AC bonus the Barbarian has +8 natural AC and +5 dodge AC when raging, for a nifty +9 AC, allowing to go two-handed while raging without issues.
So ... what ? Having physical DR is still better than none ? As you point you can also use items to reduce elemental damage, but that's true for everyone.2. DR is good against physical attacks, but some early and high level characters like the ooze and giant rely more on elemental attacks than physical attacks. I'm not saying DR isn't good, but in the long run you are relying on the same items as Fighters, which is to reduce elemental damage.
I wasn't even talking about Northlander Hewing but like you pointed Barbs get high AC when raging, another notch for Barbarians.3. Yeah, I read up on that Northern feat and it is access to everyone as I playing a bard when I saw the feat in my general feats selection. Fighters can do both one hand and two hand.
On thing you didn't mention is Ww Frenzy Barbarian get +7 to 9 extra AC in the long run, if they get epic rage.
This almost never happens in PvE, swarms of enemies mean 3 enemies and that's if you screwed up also monsters have high HP so you will land several expose weakness before actually killing one monster, making the feat that more valuable because most of your attacks will hit.Expose Weakness is good for one on one pvp, but -3 AC and period damage against hordes of enemies? Great Cleave> EW