Assize is arguably one of the most powerful AoE attack spells in the game due to both it's Potency and Range so it's generally best to use it as an offensive skill with MP restoration than a healing skill with MP restoration. However, the best WHMs will know when to use it as a healing abilities. Just a matter of practice and understanding the fights you'll be participating in. You'll most likely use it more as a healing skill in Savage tier Alex than anywhere else.