Ethereal weapons will have +50% base damage, ethereal armor will have +50% base defense. This is a huge boost, but comes at a cost of being unrepairable. However, when a mercenary uses an item, it will not use durability and is technically indestructible. Secondly, there are runewords that have attributes like Indestructible or Self Repair (f.e. Exile or Breath of the Dying), these runewords are also best in an Ethereal base. On top of the defense or damage boost, Ethereal items also reduce Strength and Dexterity requirements by 10.