I concur with Montesque's take on wight intelligence. It makes sense. Zombies, Skeletons and other "low functioning" undead are just capable of performing simple tasks. march. turn. charge. kill. however, Wraiths and Spirits are a different story.
A wraith is an ancient sorcerer who's strength of will allowed them to harness the undivided winds of magic. these winds preserved the sorceror's spirit and consciousness, but not their body. so, in my opinion, wraiths & banshees are "high-functioning" undead, with powerful wills and lingering souls, just no body to keep it in. they hang around a place familiar to them, preying on those that come by, until a vampire draws the wraith out and enslave it. they don't like this, and try to escape. this is representsd by crumble tests, i.e. if the general dies, the enchantment binding the wraith to the Vampire's will dissipates, alowing them to return to their lair.
Spirits are different. a spirit is a dead person's soul that had moved on into the afterlife and was called back by a necromancer or vampire, or was prevented from doing so by a powerful cremaining connection to the world of the living, or "unfinished business". these spirits are semi-inteligent, feeling emotions such as pain and loss, but lacking the cognition to process these emotions. so they are attracted to strong sources of emotion, like moths to a flame, trying to get at all of that delicious life force and emotions, not realising or not caring that their actions kill people.