Let's dissect what you said.
>There are spell defence spells
Yeah, there's ONE spell that reflects damage. First, your mage is never going to get targeted, and second, it takes as much time to kill a mage as to cast this spell. There are no layers of protection, which is a huge problem.
There are spells that increase DT for a short duration to match a warrior's DT. But there's nothing like Stoneskin, immunity to normal/magicalal weapons, invisibility, etc. There's nothing that would give a wizard at least 15 seconds of respite and allow him to cast a few spells before becoming vulnerable again. And worst of all, those DT increasing spells can't be cast before the fight. You open with a mage - you're dead. You can't even prebuff yourself with a few DT increasing spells and open with a mage. So you never open with a mage, and once you open with a warrior, you never have to cast any defenses on your mage anyway because the AI won't target you. It's sadness...
> there are a number of life drain/theft spells (necromancy).
Exactly, so instead of this list of spells: Animate Dead, Command Undead, Disrupt Undead, Finger of Death, Ghoul Touch, Speak with Dead, Symbol of Death, etc
You have this list: drain hp.
Steal a spell is not necromancy, but ok, let's discuss it as well: you're probably never going to steal a low level spell, because it's useless tactics-wise. Stealing a HIGH level spell could be interesting in a long fight against a high level caster, like a lich. But there's no such thing in PoE, because see point 1 - mages have no effective defensive spells.
Drain is useless, considering that it's got lower damage and your mage is never in need of hp anyway.
> enchanted weapon creation (a staff and a second higher level weapon)
You can't give them to anyone, and you can't summon them before the fight. And during the fight they're a waste of time, since they damage less than magic missiles. Again, a useless spell that was made for the sake of making some spells.
> Teleportation (combat)
K. Thank you Josh Sawyer.