Where can one find those spells?
I have never seen this happen before. Even when the body and the weapon fall away from each other, once the reanimation is complete, the weapon "teleports", or as another poster said, "snaps" back to them, equipping.
As far as necromancers go, I believe even a "pure" necromancer would still utilize offensive spells. http://www.youtube.com/watch?v=xHrlXE2PhBE showcases a build that utilizes frost spells from Destruction. Trying to think back to any necromancer I've ever played in any previous RPG's, necros often utilize poison or "absorb" health spells. Unless it's a specific build you want to play, not using any offensive spells or weapons, a pure necromancer can very easily use some Destruction spells. If you wanted to use pure Conjuration though, it's gonna be a very tricky play through.