First of all, it makes Monk/Barb way more durable, therefore they can invest 1-2 skills/passives to increase their damage output.
Secondly, we have basically no idea in what way AI will behave in later difficulty levels. What it means, is that monsters might not pick closest character as their target. Then what? Wiz/DH/WD get screwed. They don't have a 30% dmg reduction. They have to make up for it, just like melee had to do (based on anecdotal information).
And last but not least, 30% just begs for "potential of gamebreaking defense stacking". Meet Gutripper, the Invincible (crap build, i know). Just stack resistance on this rugged bastard and nothing will touch him. Sure, he won't deal much damage, but he'll stun, he'll nullify damage taken and he just won't give a f**k. Hell, make his Threatning shout use "Demoralise" and you have a cookie cutter inferno tank, without which no inferno group will be made. And that's not touching the monk. How's that for not being
Really, one of ElementalEight's suggestion would do much more good. And i don't mean it in any demeaning manner. A simple option of being able to micro out of enemy swings would be waaay better. Hah, that would actually require some skill.