That's just the thing, Cure III, while it may look pretty, is not necessary.
Let's use your example on ultimates. What's the difference between preshielding and using a Helios/emergency succor and using a cure III to get everyone up? They both amount to the same thing, with the same effort being put into them.

I can see if right after the ultimate happened another huge aoe hit that required everyone to be topped up in literally seconds. But that's not the case.

Now let's look at the case of AST and SCH effects on ultimates. With luck, a SCH can prevent dmg from the ultimate completely. For AST, collective unconscious puts up a potent shield and regen that makes casting Helios unnecessary.
In the case of WHM, I had people flat out die to an ultimate because we had no mitigation for them. It's really sad, I see some WHM throwing down asylum just to pretend they're actually making a difference during Ultimates...