Light is used to cast antimagic on the most dangerous unit in your army... The Energy Rune is essential; without it, the Gnomes wouldn't reach the enemy, except for the Trolls and their upgraded versions...
I agree that one Necropolis isn't enough because it overwhelms the enemy with numbers and darkness or summons...
Give the Gnomes a couple of castles with light magic, and the Darkness faction will have a hard time (we have Eternal Light)... and if we find another Gnomish city... God forbid!!
For the Necromancer, it's better to have Luck + Death's Grasp; that's much more effective! Leadership should only be taken if you've carefully considered the inhabited mines!