From RAW, this.
And it might not seem like a lot of benefit, but THEN I realized that with the Master of Rot and Ruin, that PP could potentially throw six dice at prayers every round (could also reroll successful ones, if he wanted), thus quickly (statistically in the first turn) triggering a Great Plague and getting Neverplague, which is gonna be really nice if you happen to have a second (or even more) Furnaces ...
For me, even this niche case would probably worth it, because being able to reroll prayers for all Furnaces throughout the game is gonna be really nice.