In case you haven't noticed, every healer has an infinite-target AE heal that's very powerful if you can get everyone to stand in the same spot. Disc's is significantly stronger in that it doesn't diminish past 6 targets and scales with the incoming damage, but also has a significantly longer cooldown. So stacking discs and rotating barriers won't trivialize AE damage vs. a 'normal' comp, because synchronized efflo/sanc/radiance/healing rain on top of the limited-target AE heals that those specs bring won't be all that inferior (and in fact might frequently be superior depending on the nature of the damage) to a single barrier.
You are comparing apples to oranges here. The efflo/sanc/radiance/healing rain are infinite healing targets, but not infinite healing. Powerword barrier is infinite targets for infinite damage.