Didn't you know that divine energy is jagged and causes bleed?

They forgot to patch it out.

To make any spear that isn't ornstein's spear or the demon spear actually sorta worth using

^ What? There's lots of good spears in this game. In fact, SKS is definitely better than the Dragonslayer Spear, simply because the latter requires a crap ton of stats.

Bow damage comes from the arrow, not the bow. Standard arrows don't benefit from divine damage. You could try moonlight arrows, but I think those benefit more from intelligence.

skeletons are also very resistant to pierce damage. you would be better off with a raw mace

Well, even if they are all called demons, the crow demons are actually pretty different from the other demons. First, they are from The Painted World of Ariamis, a place that, from a design perspective, let them put misterious enemies that wouldn't fit in the rest of the game. That said, the Crow Demons are also named Harpies and Crow People of Velka, and so also are Velka's followers or minions. Velka is the Goddess of Sin and her cult is mostly found in this world, but Oswald of Carim serves her. The other Demons are a clearly particular race born from The Bed of Chaos, who creates demons and deformityes. Also the Crow People drop soft humanity, wich indicates that they where humans, like other enemies that do the same. So they don't seem to have a conection with The Bed of Chaos, except for being also deformed by a superior power. (We can also talk about the differences between their crowlike deformityes and the insectlike deformityes of the Sisters of Chaos, or the cold of The Painted World of Ariamis and the hot Izalith. Also they would be the unique humans who turned into demonic beings.) So, they are related to the undeads of Nito? He already have lots of undead humans, and wheel skeletons can also be found in The Painted World of Ariamis. They would be the unique undead with meat, ironically. The thing to note, importantly, is that seems to be a difference beetween undead and hollows; hollows may have an undead curse, but don't seem equals to Nito creations, and they don't get extra auxiliary divine damage. With all being said, them being undead, in any of the ways, is pobably purely speculation and is not found in the lore. So why do they get bonus damage? Well, the reason is not clear, but we can see thanks to Rhea de Thorolund and the lore that having the curse and being undead is a really big burden. Religion in the lore also seems to be abused by the gods. And we see Rhea having to search for the Rite of Kindling, and fight probably a lot of undeads along the way. So the Divine extra damage probably has to do with some of that lore based Faith missions. And why the Crow Demons? Well, they are the servants of the Goddess of Sin, so maybe is a hint of an actual hate or conflict from the other Gods and Velka; and Velka seems related to the crow that helps you escape the Asylum, the sins of the Gods, the occult and extra damage to Gwyn's Knights and she seems to be secluded in The Painted World of Ariamis.

black knight weapons deal extra damage to demons

it... clearly says on the page what enemies it does extra damage to...

Yup, it does

Why should it? They're human.