Damage reduction is not affected by exo stat the same way offense is. The biggest penalty defense gets is the penalty towards vitality and spirit (which add a very small amount of defense and magic defense. But the damage reduction you get off of armor (whether it's def/mdef or flat % reduction) is not affected at all by exo. Offense on the other hand... in almost all cases uses your str and int. And your str and int are affected by exo. So you'd be missing a large chunk of your damage if you go in without exo.