I think that something else that should be considered is the party composition. I hate to keep using T9 as an example, but it's my go-to because I know its healing mechanics better than other instances. I'm...not exactly proud to say that I was stuck there for that long, but oh well.
As I mentioned before, we found that having two WHMs would, in most instances, become rather problematic. It wasn't so much an issue in T9 so long as we had a Warrior tanking for us.
With Warrior, I found that, because my WAR was taking so much damage so consistently on top of players getting hit by mechanics, my MP would often fall to about 60%-70% before the Golem Phase of the fight. I would actually use that time to let it regenerate while tanking a Golem. With another WHM, however, MP wasn't really an issue. Assuming AST is in Diurnal Sect for this, WHM could benefit from it. But the healing potency wouldn't be as high as having another WHM, so they can't answer the MP issue as well as another WHM could have unless the can consistently draw Bole.
For this, I would much prefer to see Nocturnal Sect.
In the same fight, we later had my WAR switch to PLD. He wasn't happy about it, but it made the fight a lot easier. Healing him up was no issue at all. The only issue I would encounter would be if he took a hit he wasn't supposed to, or if he got a critical. It was rare, but it did happen from time-to-time, and because I would often be dropping a meteor somewhere or would be in the midst of casting Stoneskin, Nael's next auto-attack would kill him. Two WHMs or a WHM and AST in Diurnal Sect simply could not heal through this. A SCH or Nocturnal Sect would be needed to mitigate some of the damage and allow for enough time to heal.
Note: At one point in my earlier post, I accidentally said "proactive" rather "reactive". Sorry about that.