I play mainly against the Priestess, so here's my spin on it...
While it's true that you can zone attack the temple and the clerics, the Priestess has the spells to deal with zone attacks.
Sacred Ground is a very efficient way to deal with them, for example; for only 6 mana (5 with her ring) you give Aegis 1 to living creatures in the targetted zone, reducing the total number of dice by a good bit.
Temple of the Dawnbreaker also helps too, letting you reroll lethal dice or high effect rolls (very, very annoying when I roll 11s and 12s for 2x burn...).
Holy attack spells are also the most efficient mana-wise in the entire game for dazing your opponent; with her Dawnbreaker Ring, her Pillar of Light has an 80%+ chance (still a 75% chance without the ring) to daze for a mere 5 mana. With how expensive zone attack spells are, being upwards of 9+ mana, you really do not want a 50% chance to waste you mana.
She also has Group Heal, even if you do successfully cast your zone attack spell.
The way I see it, the Priestess is similar to the Wizard in how she wants to gain advantage. A lot of her spells are geared towards gaining a large mid/late game mana/combat advantage, which is when she is strongest with her assortment of temples and high-powered angels (one of which punishes you for slaying clerics). The Wizard and Priestess, imo, are the two control mages in the base game while the Warlock and Beastmaster are the two aggro mages.
Sure, you could opt for an early-game slugfest against the opposing mage (being on the receiving end, it's actually more effective than you'd think if you're not prepared), but I don't think the Priestess or Wizard does that as well as the other two. Arcane and Holy creatures lack the piercing needed to bypass early armor (Brogan as a notable exception) and arcane creatures in particular usually have the slow trait, so they're not good at attacking outside of their range/initial zone anyway. Besides, those Knight of Westlock are probably the best guards in the game, so I don't see the reason to go aggro with the Priestess early on.
Granted, I nearly got the crap beaten out of me against two Knights, but when push comes to shove, Slayers and other demons are better attackers.