It doesn't matter what bonuses are on a piece of equipment; if it is good enough to use and has an enchantment slot, there is a selection of what you can add to it. If you need melee, ranged or magical protection, then add that enchantment. If not, then the Magic Damage or Retribution auras are not that wonderful, either.
As for the rarity of malachite gems, that is the exact same situation for every single level 10 enchantment in the game, so it is irrelevant to whether Life Line is useful, or not. The only level 10 enchantments I used were for healing auras, which always require a malachite gem anyway.