Light for the Necromancer.

YES, light is absolutely not useless, especially since there are two types of magic. However, with sorcery, you can defeat any mage, you just need to develop a counterspell.

Every mage should also have sorcery. Counterspell is very useful if the hero casts spells much more often than the enemy and has a lot of mana. Otherwise, both heroes won't cast spells, and the one with the stronger army will win. Besides, spellcasting units can absorb the counterspell.

And here's another mage who, with some probability, can develop empathy and re-cast a Necromancer. For example, an elf with chaos, sworn enemies, and new druids.