If the spirits want to be in a body and walking around, they will stay. If the necromancer took unwilling spirits; they would certainly depart. So it is world dependent: high proportion of evil people: high proportion of undead staying. Also the type of afterlife might be important. As with the Greeks; everyone goes to Hades: a very dull place; so there you might have a high proportion of willing spirits. If you are dealing with a Christian type afterlife; the bad people will want to stay alive, clinging to every thread and body, while the good people, who get cloud sitting and singing psalms, would prefer to go there and will depart immediately.
As for the life draining; The Necromancer must simply order his undead to stay out of a certain radius and not to touch him. Those released, however, may try to get revenge. For a spell that might be appropriate: check RMU spell law: Necromancy Mastery