I agree with that but what about Riev from Sacred Stones. From what I gather from the story line he was never what qualified as a "good" person.
About Biship Oro in Sword of Seals I think he kept his magic either because he was too powerful for the spirits to break the connection (as with Kenneth) or in his mind he still believed he was doing good and so that increased his power
The reason people thought Micaiah and Soren were spirit charmers was the brand, and I'm not sure about Dark Tomes being so rare, I mean once you get Pelleas the shop has dark magic to sell, and in Blazing Sword and Sacred stones as soon as you get a dark magic user almost all the shops/vendors sell dark tomes