Contrary to popular opinion, spirits are neither omnipotent nor omniscient. They can and do give wrong information and fail in the tasks given to them, things that would be impossible if they were truly all-knowing and all-powerful.
Spirits are also limited in what they can do. Most cannot do even the simplest things, or give knowledge, outside of their particular office. However, they are very good at whatever their office is, which is why we call upon them.
In my opinion, the greatest power a spirit has is multi-dimensionality, meaning they are not bound by space and time, which is why you can bind a spirit to a ring, and at the same time another magician can evoke that very same spirit on the other side of the world.
We are the only beings with the potential for true omnipotence and omniscience for we carry the seed of the Eternal within us. From the singularity of the physical plane, we are truly unlimited, and can expand beyond even the most powerful of spirits (there are planes above where we can Soul Travel, but demons and angels cannot follow).