Revisiting intelligent spirit "magic items", I like the idea of using the ego conflicts with magic swords from AD&D as an element of the system. In those rules, the force of personality of the wielder of the weapon is equal to INT + CHA + level, with level decreasing as the character is damaged. The sword's force of personality is equal to INT + EGO.
I like that well enough, though I think I'd add a d6 to each side to add some randomness during a conflict. Then I would add to a bonus to spirits if the magic-user has bound more than one. Maybe a bonus of +1 for each bound spirit to all spirits bound (bind 2 spirits, they all gain +2; bind 3, all gain +3, etc.) just to keep it scary.
So spirits would use something like this to resist control and be real dangers to their masters. Less powerful spirits might be easier to control, but have lesser powers. More powerful spirits are more dangerous to bind.
Once the binding is defeated, the magic-user might find himself betrayed, attacked, dominated or even possessed, and might have trouble regaining control.
I would also think about giving the spirits powers along the lines of AD&D artifacts and relics or intelligent swords, with minor, major, and special purpose powers, perhaps. And set up a system for determing EGO based on those powers and the intelligence of the spirit.
Low-level magic-users would probably be wise to avoid spirits of all but the weakest sort. High-level magic-users might bind some more powerful spirits, but risk danger if they are injured or weakened.
I feel like that is beginning to approximate a system.