I am not saying that necromancy doesnt have useful spells, just that starting off as a necro/death knight is particularily tough, as you lack any decent spells early on. The soonest you can get vampiric draining is level 3, IIRC, and it's not that good to begin with (close range, not too damaging...). Zombie spam is quite sweet, but you have to be at least level 4 (5 if you took raise skeleton, which is a must-have for early survival, bad as it is), and you have to get the corpses somehow, obviously. I got there right now, but mostly because of blind luck (right after spawning I stumbled upon a sabre of draining)
In my best Vampire necromacer run follower of
Kikubaaqudgha (mostly for the corpse it provides then for the Necronomicon without having to search for it), i found some of the necro spells to be awesome
The summon skeleton and undead are rather useless as offensive summons, they're just usefull when you attract someone in a floor and bring it with you to the previous floor, with your zombies waiting around the stairs.
At low level, Pain is what to use mostly even if it can damage yourself, when facing some possible threat at level 1 it serves its purpose.
Now Vampiric Draining is a truly excellent spell, of course not if facing demon or undead, it saved my character a lot, allowing me to kill some ogre when i was not yet at a level that could take them directly.
The dispel undead spell is usually great, especially when going through Crypt it does wonders.
Twisted Resurrection has been my favorite for a long time, as it is the 1st necromancy spell that allows the summon to actually follow your necromancer through stairs.
And once you get your hands on the Necronomicon, Haunt is nothing short of fantastic, this is the spell that allowed my character to defeat the Ancient Lich in Crypt, very powerfull summons though temporary unlike Twisted Resurrection, they do wonder as you need no corpse at all.