What terrible things? I need specific examples to figure out where that's coming from.
Thieves wouldn't add anything to gameplay except, possibly, flavor. There are 90 levels that do not require anyone to use ultra-special 'thieving' skills. That isn't going to change.
Necromancers are problematic for several reasons, the primary one being that souls return to the Lifestream after death.
My favorite Necromancer in literature, the infamous Yiling Laozu, required massive amounts of Resentful Energy in order to guide the dead to do his bidding. That's just not available due to the Lifestream.
I also can't see how an "oooh, oooh, I can manipulate dead bodies with magic!" would be thrilling gameplay without the souls that come with them. Especially in the arenas where we tend to meet bosses. Imagine running through one of the current 24-man instances looking for dead bodies to manipulate ... yeah, not going to happen.
