From an actual gameplay standpoint the current zombies are annoying but they are essentially placeholders for future improvements. If it meant cutting down on the no-skill campers who post up near spawn points and shoot anything that walks in front of them with the massive arsenal they acquired through server hopping, I say keep them permanently. Hell, I'd give the Z's the ability to fly and shoot fireballs. However, they do tend to break immersion, but not really any more than 3rd party voice chat and server restarts.