Diev + Miko is useful in situations whereby the targets have really high HP, are relatively immobile, and takes a while (> 1 minute) to kill. Diev + Miko will start out with relatively lower damage than most classes and can only slowly build up damage over time when statue cooldowns expire and clap allows you to build more statues. Ausrine also has a buff limit so it may not be able to provide full invulnerability for the entire duration if there are multiple players. There is also World Tree which is great for crowd control on mobs vulnerable to silence (which is almost everything except bosses).
Chaplain is useful in situations whereby you have many targets with low/medium HP. Priest mass heal, revive and resurrect are great party skills that are good in emergencies which tend to happen a lot at rank 8.