They literally stink.
But seriously, the AoE poison is good for grouped up enemies within 1 square of the Zombie, and the explosion deals a big chunk of damage to the affected targets in range. Quite a few bosses are either too big or keep you at a distance where they're not as effective, but as someone levelling Slayer, I've found that the Zombie is good to have to get your DPS up when targeting a few of an enemy type. I alternate between Zombie for damage and Ghost for mild sustain (though if you're surrounded by enemies, Blood Siphon will offer better healing on a long cooldown).
What are you fighting at the moment that you want to bring the Zombie into the mix against?