RQG p 249: Those in physical contact with a binding enchantment can mentally communicate with an entity bound inside (if there are no conditions to the contrary) and can command the entity to use its abilities. ... Some entities have knowledge or abilities which the wielder can use while it is bound within an item. However, many entities are not very effective when so trapped and must be released to be useful (e.g., wraiths, healing spirits, elementals, etc.).
I understand that if a spirit knows a spell and is bound in an enchantment and you are in physical contact with the enchantment, you can command the entity to cast the spell. In addition, (RQG p366): Spirits may be bound into a magic crystal, or into a specially prepared object or animal as described in the Binding Enchantment section (page 249). The binder of a spirit can use any spirit magic the spirit possesses and the magic points of the spirit to fuel spells.
I would very much prefer to have this situation and would be very happy if I'm wrong. On that point, I prefer the different RQIII types of spirits.