Disc priests using tons of PoH are far weaker than a PoH spec (or even Renew / GH spec) priest at the job, but I don't think many people consider that a viable primary tool for disc. The area they shine most in for raid healing, outside of LK, is preventing deaths which would be avoidable anyway if dps were paying attention - such as not slow-dancing with ghosts on LDW by selective 'just-in-time' shielding. Other than that (which happens frequently enough even to the most dedicated players that its value should not be dismissed lightly), their greatest contributions are a mild dps-enhancing cd and the 3% overall damage mitigation which is moot a prot pally able to sanc the raid.