H2 priests: Why, when, how?

[JimMorrison]

Also, because of the way national casters are designed, it's true that few of them fall under the "aren't useful except for Sermon" clause.

Just 2 of the last 3 direct examples - Caelum's Seraphines are stealthy, and can fly. And Marignon's H2 is an Inquisitor, no?

I think Sermon is of rare actual use in expansion (if you are suffering much attrition, you are probably doing it wrong to begin with!), but it definitely is of great value in those BIG fights, where a large portion of what decides the battle, is whose line starts breaking first. When your more timid chaff breaks too early, all your elites get swarmed and chewed to pieces - and you wasted a lot of buff casting. Some nations in particular have mages more suited to casting little spells over and over, rather than dropping a few Thunder Strikes, and napping. With an endurance army, Sermon is certainly of more value than with a shock (no pun intended) army.