I always thought of Necromancy as redistribution, strengthening and sustaining of Life energy through (arcana? incarnum? mana?) whereas resurrection is the reconstruction of a body so that life energy flows naturally (which would be why death from old age would be unavoidable).
That's what Ketham does: he takes life energy from those he kills (or himself, depending on the spell) and uses it to fuel his spells. When another is involved, it's usually in the creation of undead, as he has unlimited life energy due to a specific event in his past. It's one reason he can't use anything but necromancy: he just isn't able to "map" - for lack of a better phrase - the energy properly.
I'm trying to distract myself at the utter abomination i have created to represent DBright until i fix it =(