Having used undead recently, I can say they will chase them for some distance. So you can pull NPCs away from areas or herd them somewhat into a group if you do it right. I used one in the Grymforge area near the trapped Nere to herd up most of the dwarves there into a cluster and then dropped a fireball on them. Had them all standing together trying to 'shoo!' my zombie and then, boom!
