I've never quite grasped how necromancy could be useful. Since you have to pay the full costs, then you're not saving any resources. I suppose it saves you card draws, but not having enough cards is rarely my limiting factor (what I put in to defend the quest zone usually gives me enough cards). Also, I suppose you could put in fewer units overall and have more space for tactics. Still, having the unit disappear at the end of the round is a pretty hefty price to pay that doesn't seem to outweigh the (minor) advantages.
Still, people do seem to like it, so I figure I'm missing something. Why do people like necromancy?