I chose Necromancy because the Dunmer seem to be more skilled in the darker forms of magic, and even a heroic character would be willing to raise corpses to help him fight against evil. I saw it as giving the raised dead a "second chance". Of course you could use Atronachs, but the most fitting one, the Flame Atronach, is also the weakest whereas raised corpses can be up to around level 30, which allows you to raise most enemies.
I chose Necromancy because the Dunmer seem to be more skilled in the darker forms of magic, and even a heroic character would be willing to raise corpses to help him fight against evil. I saw it as giving the raised dead a "second chance". Of course you could use Atronachs, but the most fitting one, the Flame Atronach, is also the weakest whereas raised corpses can be up to around level 30, which allows you to raise most enemies.