I've bounced between Holy and Disc through all the expansions, based on which was viable. Often I had two healing specs.
The main problem is that Disc shields completely snipe heals. So, instead of having bubbles absorb ALL damage. Have the bubbles absorb half the damage, and let half through. PW:S could still absorb 120k, but if an attack did 100k, only 50k would be absorbed, 50k damage would go through, and the shield would still have 70k left.