The Spirit of Oppression can be very useful when your army has no morale-boosting skills or items, and is especially useful for Necropolis (undead always have neutral morale) or Conflux armies (5 of the 7 creatures in the Conflux also have fixed neutral morale). Against neutral stacks it will prevent extra morale-based moves which may thwart an otherwise perfect tactical plan (neutral creatures subject to morale will always be +1 otherwise).