Necromancers have little mana, but they can easily replenish it in battle, and they have various artifacts. If they also get an education, they'll have enough mana to cast spells. Dark Elves also have little mana. The point is that light magic is not needed by necromancers.
Do you understand what you wrote?
1) Necromancers have little mana, it's easy to replenish, they have various artifacts, get an education... and? :confused: ... enough to cast spells for Dark Elves!? :D
2) How did you come to the conclusion that "light magic is not needed by necromancers" from what you wrote above?