They wouldn't be quite so much trouble if the DM added the Vampire's assorted weaknesses (Garlic, Mirrors, Holy Symbols, running water problems, need permission to enter homes, dies fast in sunlight) to stave that off.
Of course, the D&D dieties might also take notice and deal with a sudden Wight swarm themselves.... hmm.... could make a good plot hook - the cleric's diety send him a vision, and sends him on a mission - he has to stop a magic user from enervating his attacker to death... but the attacker is disguised, as is the mage....
For a hack&slash group, they can just track down the event, and deal with the body when the time comes. For an RP group, they can see about arranging for the attacker and mage from ever meeting....
Back on Topic:
For the most part, necromancy is a rather dark art, even those spells that don't have the evil descriptor. That said, it also has some really useful spells that don't hurt anyone meaningfully (at least, not directly - Spectral Hand can be used to deliver damaging or status ailment spells, but also good spells from other disciplines, up to 4th level, anyway) such as False Life (shield yourself, sort of, anyway), Spectral Hand (increases the range of what is meant by "touch" ... for spells of 4th or lower), Gentle Repose (useful for preserving the body till you can do the next on the list, if you don't have access to clerical abilities), Clone (raise dead!), and Astral Projection (explore the Astral Plane!). The rest either deal with undead directly, deal damage, or change other's status for the worse (for them). Of course, other schools have much worse stuff, if you ignore the descriptors - for example, Trap the Soul is conjouration, Feeblemind is enchantment, Imprisonment is Abjuration, Baelful polymorph is transmutation, and Phantasmal Killer is Illusion. Should the school be judged on the fact that it mostly focuses on hurting your enemies? Granted, it contains some of the more torturous spells (assorted undead-making spells), but it also holds some protective, restoritive, and utility spells as well. How many other schools accessable by a vanilla Sorceror or Wizard permit the Sor/Wiz to restore a fallen ally to life?