[Agema]

True, the majority of battles are sufficiently one-sided so that +2 morale may not be here nor there. Although as pointed out, if you've switched the dom on a province to yours, you'll be fighting with +3 to your opponent's -1, (as opposed to +1/+1) and that's beginning to look like a big advantage.

Priests should not be sitting around idly. They should be busy preaching every turn they aren't moving or helping an army. This can give you the battlefield advantage above, but it's not just that. H2s convert provinces MUCH quicker.

A province owned but with enemy Dom means suffering several of the worst aspects of their scales and none of the benefits. Convert your provinces, you get the benefits quicker, which means more gold. Not only that, but your temple/God/prophet dom will stop filling your provinces and start spilling over onto your opponents' provinces, meaning they'll start losing productivity. Thus preaching can potentially (depending on scales) increase your wealth and decrease your opponents'. Then also consider combining that with certain globals made effective by your Dom (e.g. The Looming Hell).