Game Mechanics: Caster behaviour

[Brut]

Noticed this with shin lord in uguk and the sentinel in erudin docks: they cast heals when low, but won't LoH or cast any of their stuns. Same could be said about some cleric mobs, like in Droga the clerics would buff themselves, but then only cast their heals when low, never trying to nuke. Is it supposed to be like this? Shamans are dotting and such as usual, but pallys/clerics behave like complete tools.