Yeah, necromancers are mostly used for their tokens. A bit like how Karma is used for her immortality skill. The tokens do not count if they die so they are placed to slow down bosses. It's quite good for bosses with hard hitting slow attacks since they will just completely stop them while they're charging up that next attack.

They're not witch replacements, they're additions if you want to completely stop something so you have more time to destroy it. Also good as targets for missile attacks if the enemy has any hard hitting such ones.