There is a necromancy "perk path" on the conjuration tree.
Pure necromancer would involve only using necromancy "perk tree" and skills like raise zombie.
And why would a pure necromancer not raise demons as well as zombies? You know, necromancy does not only involve raising the dead - it has to do with the general theme of manipulating life and souls. So, for one, a pure necromancer would also be a master in restoration. They would also be able to use weapons and staves enchanted with things like absorb health and absorb soul. And they would almost certainly cooperate with the daedra, thus they should be able to call them to their aid. So, a pure necromancer would certainly do a lot more than simply raising corpses.
A mage that can only raise the dead is not a pure necromancer - they are simply incompetent, like a blacksmith that was never able to craft anything other than steel weapons and armor...
Well then what is the point of necromancy if you can't purely rely on it?
Necromancy seems useless if you can just summon atronachs or use skills like fury to have others fight for you.
Necromancy is useless then?
You can be a pure archer (only use bows) in the game.
You can be a pure warrior (only use 1 type of weapon).
You can be a pure destruction mage (Thunder, fire, or ice) I've done it.
But not a pure necromancer. Nice
I repeat: there is no skill named necromancy. You can't rely on something that doesn't exist. On the contrary, there are skills named archery, 1-h, 2-h, destruction. Simple as that...