For a quote +2 morale brings:
- +20% of base attack/defense of unit, that's not that much for a low level unit like priest but it's still a good gain when effects are multiplied by 2 against undead.
- It increase the critical chance of unit of 50% (ie for a priest the base chance is 5% without rage bonus, it becomes 7.5% without rage bonus. But that base chance includes items improving critical chance).
Also another quote, even 2000 Skeleton archer will do ridiculous damages to Cursed Ghost with Stone Skin level 3 and Archmage shield. But this isn't useful in this battle because of that two Necormancers stacks that are hard to close contact both.