If your frost and have shatter Molten armor counts as a "spell" and will have an extra 50% chance to crit when the target is frozen.
This is very useful if you went some into fire (enough to get ignite *40% of critted damage of fire spells into a dot*)
Very very nice against rogues in pvp when you have ice barrier up and rogues get the opener on you. No vanish for them (after CoS is used).