What are spirits, exactly? Are they living energy that just happens to take on the characteristics of different domains and metaplanes? Are they sentient beings that are already thought out and created and are summoned with the powers they get through some magical Google search function? And why does a true name have so much power over a free spirit? And why are spirits' activities broken up into services rather than amounts of time spent?

My theory about true names is that they are gateways into the mundane plane; while a spirit has services, its conjuror functions as its gateway. When it goes free, it gets its own gateway. Therefore, if you can monopolize its gateway to Earth, you get power over it.

As for the services question, I would say that one compels services from a spirit by "nudging" that link between the metaplanes and the Earth. Each time you nudge the link, you degrade it some, and use up a "service."