Wouldn't Disc mastery scale with PoH as well, since it auto-pops DA?
This is a common mistake people make when considering priest stats. The answer is: kinda. DA procs an absorb that is 30% of the total heal, which put another way means that the shield portion of a Prayer of Healing accounts for 23% of that shield. Discipline mastery therefore -only- benefits 23% of their PoH. Compared to Haste, which scales with the entire amount (The more casts you can do the more healing + more shielding you do) and Crit which scales unusually well with PoH specifically .. instead of just doubling the amount of the heal, it actually not only doubles the initial heal but then double dips and provides a shield that is 60% of that already doubled heal rather than 30%. This means that crit provides an actual increase of 246% to PoH, instead of the 200% that it provides to everything else. This is all without factoring in Glyph of Prayer of Healing, which is actually quite important because that, too, scales with Crit, and haste, but not (much) with Mastery.
Mathematically, PW:S is the only
reason Disc priests care about Mastery, and Strength of Soul along with a desire to play to their strengths (clutch healing) does give priests a good reason to use their Mastery, but in the long run this really hurts priests because, while it's not exactly weaker (nor is it exactly -stronger-) than other stats, most Disc priests feel the need to play to this one strength of their class while the rest of their toolkit gets short-changed.