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In a long campaign, it's common to switch between different heroes, using one for the early game and another for the endgame. In such cases, Logistics might only be needed for scouting and capturing enemy units, which means you can focus on developing Attack, Defense, Darkness, Intelligence, and Light skills. If you're not relying on summoning, I don't see much point in investing in Lucretia, as Sorcery becomes almost useless. Although, you could try focusing on non-mass spells, but you'll have to sacrifice something else. While Light magic is rare for a Necromancer, it can be useful, not because of mass buffs (as there are already some), but because of Antimagic (to prevent magical races from wiping out a strong stack with spells like "Explosion," for example, against skeletons or vampires), Divine Vengeance (which is extremely effective and, frankly, almost overpowered against units composed of numerous low-level troops. A single powerful strike against skeletons equals death from Divine Vengeance), and, less frequently, Fire Protection. However, building a guild in a non-core castle for just 4 points is not efficient. Of course, if you find the "Book of Light Magic" and are offered the Light skill, it's worth considering.
However, all other things being equal, developing a Demon hero in Light magic is much better than in Darkness, except when playing against Orcs.