I'm fine with necromancers occasionally having high-level zombies, with the caveats that:
1. It shouldn't be trivially easy to get them (e.g. water sprites and undead invuln to water), and
2. There should be some trade-off, e.g. they don't stick around as long.
I also think it's kind of silly that zombies can be summoned away, *including* by people *in* the necromancer's PK. I'd either make then unsummonable *while in the same room as the necromancer*, or do something where the summoner has a chance for something nasty to happen to him (e.g. psychic vampire).