Alright, so I tried it.
Here we have my Minotaur necromancer with 2 int. I have not found dispel undead, land so the point of this experiment was not made.

Cigotuvi's putrification is very strong in theory, but hasn't been used to effectively destroy a boss or strong single monster as of yet.
Curse of agony hasn't seen any use, due to 5% power meaning it can't really overcome willpower.
Switftness was a freebie after cigotuvi, but hasn't actually been used.
Grave claw is as good of a disengage as swiftness, but also hasn't been used.

So, the bonus of investing in necromancy, spellcasting, and air magic is, in effect, wearblood/fugue of the fallen, which generally amounts to +1 slayer against less than half of targets. This lasts for basically no turns at all, but I still get it up to +7 slay sometimes.

The downside is that, normally, I would be reaching 22 fighting and 22 throwing by now, instead, which is enough that okawaru pushes that to the 27 level cap.

Yet, I am somehow alive and have cleared vault 4.

This is, simply, because I consider my character weak, and preform proper separation of the enemies before engaging. I haven't reached the point of shouting into the dark of every room before I enter, but maybe that's how the game was meant to be played.