I think it would be nice if having access to a pain-branded weapon allowed necromancy training, is there some reason why it doesn't?
Example: Okawaru gifts you a quick blade of pain, but the RNG has decided to not generate any necromancy spells anywhere.
Edit: replaced my original not-very-good-at-all example:
Okawaru gifted me the +9 quick blade of Noxkoso (weapon) {pain, +Blink Str+2 Dex-3 SInv}, but the only way for me to make use of the brand would be to learn Regeneration (the lowest necromancy spell I've found).
That would mean either raising spellcasting as well, or spending scrolls of amnesia to swap spells around (and temporarily walking around without one of the spells I'd actually like to have)