Souls, Black and White says nothing about a soul being required to make intelligent beings. It says only that "Every creature, living or dead, is powered by a soul. Without it, they are just lumps of flesh or piles of bones.", which basically just means to animate a body (permanently), a soul is required.
I find it odd how a mindless lump of flesh still needs a soul. How can something that is dead have a soul? If they meant undead, how can that same person be constantly resurrected again and again if its soul can only really be captured once and used once before it's leaked right back into the Dreamsleeve?
Even better, On the Preparation of the Corpse states that "While the Arts of Necromancy can be practiced on animals, such experiments rarely produce interesting results. The servant's ability to follow directions seems to be related to the subject's intelligence in life. While raising the corpse of a man, elf, or beastman can produce a useful servant, the corpses of animals produce mere guard dogs at best." I think this is a least decent evidence that a soul could be used in necromancy, as the servant does have a mind of its own unless they are intelligent enough to be properly controlled by the necromancer.
If that is not total evidence, at least we do understand that an undead person has some intelligence of their own, and are not completely mindless slaves. In fact, On the Preparation of the Corpse mentions intelligence several times, such as making a "mummified corpse with the strength to move itself. Most importantly, you will have a much stronger servant who will follow your commands with more independence and understanding."
First off, do you really expect an undead dog to be able to sweep the floors? A bear? Any animal in fact? Unless heavily trained, I highly doubt it.
Next, I don't see that much evidence. The necromancer makes it seem like your bodies have lost their soul way in advanced. The ability for an undead being to remain useful probably still stems from their brain having more cognitive abilities in life, which translates into the servant being more useful, as it gained some better basic understanding. Plus, how is the soul of a bandit yours when it was donated by the empire or you somehow stumbled upon a body while walking on the road? Makes no sense. If you ask me, this shows that if a person was intelligent, they had more apt brain functions and somewhat carries on when they're dead.
Plus, how does making a mummy magically makes it more "smart"? I can understand it being less likely to stink and decay, but being more intelligent? It's probably more independent due to the less chance of decay and falling apart because of decay, which makes it seem more independent.