PVE Priest healing spec... Disc or Holy?

In my mind, there is a solid spot for 1 regular disc priest in any raid. Any more than one and you're not gaining any quantifiable benefit sadly.

However, a disc priest does make an excellent difference once people forget HPS and start thinking about 'effective mitigation'.

Talented PWS from a disc priest raises the effective HP of a tank, as does Aegis. This makes tanks less susceptible to large spike damage. Maybe we're not seeing that in Naxx 10, but as bosses get harder and PWS and Aegis get larger from better gear then it will become more noticeable.

I don't mention Grace on purpose as it won't stack with BoSanct but in cases where no Pally tank is present then add that too.

Remember, if a Disc priest does 0 healing, but absorbs 40k of damage through shield and Aegis, and a Holy Priest does 40k healing, the effective damage mitigation / recovery is the same. It's not all about HPS.