Almost all the time that you could pop the sch bubble you can use CU

The difference is the cd and channeling
You could think that it doesn't help

But in fact i almost never stay in CU the whole time
Only to soak in the raid damage and apply the wheel of fortune (super strong HoT) then go back healing

As long as the CU is running the wheel is reapplied so using the CU fully and adding Celestial opposition give you a 30sec HoT on the whole group
Giving you almost nothing to heal