I never understood why Holy required so much MP to cast. I does maybe the same if not less damage than Banish III most of the time, and has one hell of a recast.

1k+Holy doesn't sound so much like good macc as it is maybe good healing numbers with Solace on. Solace only effects Holy damage, no other light-based damage spell, and your recorded healing numbers get erased with each cast. Misery increases Banish damage based on other damage dealt, though - similarly to Solace+Holy - casting a Banish spell erases that recorded damage and you have to start from scratch again.

I'm pretty sure the light-based nukes were designed just as a small damage bonus to debuff undead (for about what seems like the same duration as Flash's blindness), and not meant to cause even RDM-worthy damage. Though I gotta say, making trials that rely on light-based damage kills is a pain without any decent damaging light-based nuke spells.