90% of the fight the enemy has radiant resist so holy is nearly useless. I can't exactly have every spell caster in my party to try to have a different element every damn round. And Larelle is a dead weight the whole fight. She might as well be replaced with a dead fish. Why should I need to teach her special spells for HER scenario?
First of all why wouldn't you want to use Larelle? She's hilarious. Her Last Quest chats not infrequently change depending on whether you bring her along.
Second, she doesn't need to learn new spells for her scenario. She just needs to equip an item giving her non-Necrotic/Psychic AoE attacks.
Last, if you haven't been using her before Haven of Gloom you may want to use her after. After her scenario you might say she becomes overpowered. Her Necrotic/Psychic attacks get buffed +50% (so she can now damage previously immune enemies and woe to enemies already weak to Necrotic/Psychic). Her unique accessory is insane: combo of Rhinestone Glove (mana regeneration), Biotic Amp (insane buff to magic attack), immunities to a lot of stuff plus more. Don't forget that Larelle has HP regeneration even before that.
If you taught her spells before her scenario she becomes extremely useful after.