Doesn't it take a long time for corpses to reach the zombie and skeleton stage? You would need to have access to a graveyard or a battle site from a week or two before...
I'm not familiar with how long this spell takes to cast, but it could be handled like a prep spell -> give the player options outside of combat, tracking everything with ASLs including the date so that if you did come back to the seen of a recent battle you could raise some undead. If one had a way to track who survived on the party's side and their HP, ASLs and some scripting could keep track of how many undead a mage has at her service. It would be pretty complicated though, at least for me.
