A necromancer becomes a Lich to extend his life. Don't see why a necromancer would want to throw away his youth. He's going to have next few hundred years to be a Lich, I think he can wait.
Well not necessarily just extending their life from old age and sickness, battle wounds too. That would actually be a really cool gameplay mechanic, to respawn everytime you die at your phylactery instead of having to reload a save. And if someone does destroy your phylactery (Oooo like fighters guild mercenaries contracted to kill you or something, don't laugh too hard its totally doable), then you reload the save.
Oh I think I did play that morrowind mod a little bit a while ago, that was pretty good. I second the no hide true form though, but a robe disguising your appearence ala Gray Fox would be ok I guess.