there is this consideration as well...
A wise necromancer does not wish to fight for control of his creation with an angry spirit seeking a way back into the world. Best to be certain all of a creature's soul has departed before reanimation begins.
though I am a bit confused of the skills needed to reanimate the bones/skeleton of one whose original soul has already left... soul gems maybe?....alchemical recipes of reanimation solutions?
I've got two necromancer characters that I've had for a while, neither of which I'd consider outright evil.
One is an Imperial that made a pact with Peryite to save his parents from the Knahaten flu. Peryite agreed in exchange for his soul. When his parents ended up dying anyway (they didn't have the Knahaten flu, just a normal flu), he decided that he would never die so that Peryite could not exact payment. To this end, the Imperial studies necromancy in order to extend his life (hopefully indefinitely). While he definitely takes evil actions, the end goal isn't simply power.
My other necromancer character, which I think I'll play the Elsweyr story with, is a Khajiit private investigator that often works with the Khajiiti government. He definitely raises the dead, but mostly does so in order to ask them questions so that he can solve mysteries.
Won't have to buy more slaves if you can raise them back.