QUOTE (knasser @ Oct 31 2008, 07:15 PM)

At any rate, I don't think it's unbalanced under most circumstances for a materialisation mage to banish and control possession spirits or vice versa. The effect of this when you have a team containing both kinds of magician however, might be a bit more interesting, as they try to swap spirits with each other.
What we have to consider here is that the team in this example would have access to both types of spirits anyway.
I can imagine some examples where it would really expand the team's abilities, e.g. only one of the mages would have to pick up the invoking metamagic.
In the case of, say, a Hermetic with CHA 1 and a Houngan with CHA7, it may also be useful to hand spirits over to the houngan to have a more well-rounded arsenal of spirits at hand, as the Houngan could bind 6 more spirits than the Hermetic.
No need to run around with 1 materialization and 7 posession spirits when you can arrange something more flexible, right?
However, effects of this would be pretty minor.
Yeah, the houngan could have a spirit of man sustain 2 F6 natural spells on him.
But this would also be the case without handing over control and the spirit's spell selection wouldn't change, either.
Balance wise, i don't see much of an issue here, with very minor exceptions, the team's ressources stay the same and what little improvement is achieved is just an acceptable and certainly not unbalancing effect of good teamwork (and keep in mind that it involves buying up a skill a lot of players would normally ditch).
As far as the fluff is concerned, however, keep in mind that being banished is painful to the spirit.
Such attempts should ideally either be reserved to emergencies like averting a magical threat or be combined with efforts to appease the spirits for their discomfort, by offering sacrifices to them (above the point where sacrifices are a part of Binding anyway), fulfilling tasks in their interest and so on.
If such preparations are met, i see no problem with swapping spirits within the team.
However, deliberately giving away spirits becomes potentially nasty when you include people
outside of the team.
Say, a kind of Hermetic slave trader ring who sells spirits of man to traditions who normally cannot summon them (man, this thread gives some great ideas for my next group of magical villains).
This, however, is certainly an area where the trader (and, most likely, also the customer) run a high risk of acquiring the Spirit Bane quality or at least spirits using their Edge on a regular basis to resist summoning and binding.