Some questions.
1). If fusing with a spirit is bad for you in the long run, does simply seeking or summoning out a spirit for advice cost anything?
2). Would binding a spirit to a machine, weapon, or otherwise inanimate object to amplify it be possible? be considered amoral?
3). Faith. What role does it play when there is a defined something beyond?
4). Are there certain spirits that are stronger spirits that can burn someone out faster?
5). Are there means of forcing possession upon a person to force them to divulge their thoughts?
According to my
proposed spirit mechanism, which people seem to find agreeable thus far:
1) Humans don't summon spirits, and fusing with them is not necessarily bad in the long run if all goes well (can give you healing and shapeshifting and maybe other things). You should be able to talk to it just fine, though. I'm not sure if they're supposed to be properly invisible or just intangible and ghostlike.
2) You shouldn't be able to bind spirits to objects, either. You can try to twist them into one - this kills you in all likelihood if they resist, and they probably will, and kills them if they do not. Maybe kills them if they do. You don't really need enchanted objects so much if you have a spirit to make your body into whatever you need, though. And spirits affecting objects as such isn't something that's been addressed in my model. Let's say they can't to preserve balance, due to the object being non-living - life forms a bridge of understanding between the spirit and what it seeks to affect.
3) It plays the same role it does in our world. Faith can be a useful relationship between a human and a spirit, as it allows for the human to trust the spirit more easily.
4) Spirits and humans fusing with one another works a bit differently - spirits introduce chaos to the human body (turning their bodies into something more useful for the moment), while humans introduce order to the spirit (mostly undesirable, since this can kill a spirit if gone too far). The fusing is mostly a unique thing for each pairing, and spirits don't have power levels as much as spirits and humans have their relationships.
5) A spirit can forcibly fuse with you, but this can easily backfire on it if you know that they can be killed simply by you thinking about the wrong thing too hard in the process. It's also definitely not possession, though I could see how a spirit (an uncautious one) can try impromptu brain surgery on a person, though results are unlikely to be pretty in any way.
Of course, that's just according to me. People can oppose these thoughts, since I don't have the final say.