In PoE1 priests had strong non-spell per encounter abilities (that could be further boosted with feats).
Painful Interdiction crippled enemy defenses, particularly Fortitude. Particularly good to land some spells, like Disintegrate or take advantage of Barbarian Brute Force (target Fortitude instead of Deflection if lower) - and crit non-stop.
Inspiring Radiance is not only a nice and cheap aoe heal, but also briefly boosts Accuracy by a stackable +10.
I dropped him around level 6, so no per encounter priest spells. I might pick him up again at later levels - I'll probably end up choosing between him and Grieving Mother. No doubt per rest abilities can be powerful, but I don't care for them since it means a lot of time is spent guessing how many more encounters a game designer put in a dungeon. Always felt a bit meta-gamey to me.
Only got to pick 2 talents, so I went with Inspiring Radiance and Inspired Flame, which meant Durance was good for a one time temp accuracy boost per encounter. I should probably just respec all my companions though - I looked and the auto-level had apparently picked Bear's Fortitude for Durance's first talent when I initially picked him up.
But either way, replacing him with Pallegina has worked well enough, even for big fights. PotD doesn't seem that hard, although it does have some weird difficulty swings.