An alternative approach would be to make some area effect spells with undead themes (yeah, I saw a commercial for Zombie Tidal Wave). It seems to me the benefits would be that:
1) you are getting "a lot" of undead but aren't adding to GM headaches by having to keep track of where each of those undead are.
2) since the necromancer isn't micromanaging 30 skeletons, the necromancer's player's turn shouldn't take 45 minutes.
3) an interesting heightening effect could be that the spell leaves behind a zombie/skeleton/whatever appropriate undead when the spell ends.
4) when the party goes to sleep at the end of the day, the necromancer doesn't have to worry about a random group of gnolls or ghouls eating half of his/her zombies (for some reason the "I've got an army of undead" necromancer has never been popular at my table--it is a real mystery).
5) This becomes good precedent for other themed area effect spells (I'm looking at you Impnado or Lantern Archon Thunderstorm).