1. They are not subject to damage reduction.2. "Light" is a type of energy (see Searing Light spell description), but it is not an element. AFAIK there are no creatures that are strong against light. Some undead are weak against light.
3. Yes, they seem very good. I don't really care about their DR, but they can actually land blows in combat. I think their damage is pretty low, actually. You'd be better off casting Greater Magic Weapon on your fighter's sword, axe, etc. At least, in my experience, YMMV.
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