I keep running into Astro running Shields in Farm PTs as a SCH. (Why?)

[Elamys]

I have the opposite problem, I like to go diurnal when I'm with a WHM on easy content simply because it's more heal per cast and I have access to another regen.

But even if the boss does nothing that could make a shield remotely useful people still ask me to go nocturnal because '' shields''

Preventing damage is still strictly better than healing up the damage later, so you should be going Nocturnal with a WHM even if the shields "don't seem very useful".