QUOTE (crizh @ Mar 25 2010, 09:47 PM)

The simplest reading of the Possession and Vessels sidebar is that the values of the possessing spirit's mental and special attributes are used as the values of the mental and special attributes of the combined being that results.
While it is certainly possible to read 'the spirit's Mental and Special attributes are used' as meaning that the combined being actually doesn't have it's own Mental and Special attributes but the part of the Spirit that possesses those attributes remains separate from the combined being whilst still controlling it, that is only one possible meaning of those words and I put it to you that this is a stretch at best.
A combined being is a combined being. There is no Spirit and there is no vessel there is only the combined being. Whether or not a particular attribute is the spirit's, the vessel's or both combined doesn't matter. That is the value of that attribute the combined being uses for all purposes. When you introduce channelling and the combined being has two minds that both share control of the combined being they both use the same attributes for all purposes.
Unless you choose to interpret channelling as so obviously using the Magician's Mental and Special attributes for his own skill use that it doesn't need stating, then the Magician's skill use benefits from the combined being's attributes for all purposes.
There is no point repeating that the attributes used are the spirit's. Mechanically the spirit has ceased to exist, there is only the combined being.
You are placing way, way to specific a meaning onto the word 'used'. Without further clarification in the text that supports that interpretation you simply cannot exclude all the other possible meanings of the word to deny a character access to so fundamental an ability as Edge.
SM pg. 95
Spirits and Edge
A spirit is generally under the control of the magician who
conjured it, but to one degree or another it is still an independent
entity. Even while bound and compelled to obey, a spirit
has its own fate and its own free will—as such, a magician cannot
compel a spirit to use (or not use) Edge on a given test.
Spirits will likely use Edge to save themselves from disruption
or banishment, or to assist with the completion of a goal important
to the spirit or if completion of a service demands. Any
use of Edge is at the discretion of the gamemaster.
Spirits can also use Edge to assist their resistance roll to
the original summoning, but will generally not do so unless
the discrepancy in power between them and an impudent
conjurer is large or the conjurer has a history of mistreating
spirits. And yes, spirits do know if a conjurer has mistreated
other spirits. Whether the rumor mill in the metaplanes
works really fast or spirits can somehow pick up the telltales
in a conjurer’s aura, the spirits know if a magician’s been bad
or good.
DG pg 10
RESOLVING POSSESSION
Th e following notes are meant to complement, clarify and
illustrate the rules on spirit Possession, p. 101, Street Magic.
Spirits conjured by Possession traditions are intended to
be as easy to manage as spirits conjured by other traditions. Th e
only signifi cant diff erence between Possession spirits and materializing
spirits is the manner in which they bridge the barrier
from astral to physical. Otherwise, such spirits behave just like
other spirits in all respects and the rules for spirits apply equally
to spirits with Possession.
It is important to keep in mind that once possessing a vessel,
the spirit’s mind and not the vessel’s is in control, and the spirit
does not have access to the vessel’s knowledge or skillset.
SM pg 54-55
The Channeling magician can use her own skills and has fine motor control
over her body while enjoying the enhancing benefits of the
Possession power (see p. 101). Control is still shared, however,
and the magician is unable to tap the possessing spirit’s
powers without expending a service. Additionally, the vessel
resists any mana spells or powers with the lowest Mental attribute
of the two minds (whichever is lower, the spirit’s or
the magician’s). Otherwise, resolve the effects and duration of
Possession normally (see p. 101).
SM pg 101
Living Vessels
If the vessel is a living creature, the spirit’s
Force is added to the vessel’s Physical attributes.
While possessed, the spirit’s Mental and Special
attributes are used (which means that a possessed
technomancer cannot access Resonance),
with Initiative recalculated as normal (use the
spirit’s normal Initiative Passes). The spirit is in
full physical control of the vessel, but does not
have access to the host’s knowledge, skills, or
experience. The mind of the vessel remains in
whatever state it was when possession began;
if conscious, it becomes an impotent witness
locked inside its own body for the duration.
Possessing spirits cannot perceive or operate
AR or direct neural or cybernetic interfaces,
and do not benefit from implants, cyberware,
or nanoware that would require active control
(i.e.: a spirit can benefit from a vessel’s bone
lacing or eye replacement, but cannot activate
vision enhancements or a datajack).
--
You can't make a spirit use its edge.
When possessed your *special* attributes are the spirit's... not yours. Like the SM says, you don't have access to your special attributes otherwise a possessed technomancer could use Resonance. Since you don't have access to your Edge you can't spend your Edge. The possessed vessel is using the spirit's Edge so only the spirit can decide to use Edge.
You lose the ability to access Edge but you gain the ability to use the spirit's Magic (not to mention the other perks).