It's tough to build high level armies until you have greater undead and even with the couple of tricks before that getting those corpses sub 40 holds an element of risk.

The level of the caster Plays into the level of the zombie so the old dwarf of a level 30 necro is not as potent as a specter.

Also while zombies wreck house in the mid tanks they play a much more limited role in high level PvP. I think they are well balanced at 42plus but kind of overkill up to that point. Unless there are summoners totally ruining the necros day.