So what about the materialized spirit in airplane form? Or the spirit in Knight with a sword form? What you describe is more or less the function of a vessel for possession / inhabitation spirits.
Lets take a look to all the rules which govern materialized spirits
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Spirit Forms and Combat
Spirits materialize in all manner of forms, including those equipped with weapons such as swords or claws. Th ese “weapons” are (obviously) part of the spirit’s materialized form, however, and do not provide any extra bonuses. A guardian spirit who materializes with a sword in hand, for example, does not gain any benefi ts of a sword’s Reach, nor does the sword deal any more damage than the spirit’s materialized fi st.
Its very clear: a spirit can look like anything it wants (and fits the them of course) but there is no change in function: you are a spirit, strength / 2 in melee combat, 3D movement ability for all materialized spirits (as defined in the basic book) dual natured, improved natural armor when materialized etc. I hope that there is no misunderstanding or contradiction in that.
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SPOTTING SPIRITS
The spiritual nature of any materialized spirit form is unmistakable. Even if the spirit materializes looking like a toaster or a Lone Star officer, any sentient observer will have no difficulty understanding that they are observing something unnatural.
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While a materialized spirit can’t be mistaken for a material creature or object, it can look like almost anything, depending upo n the tradition responsible for its onjuration (though a particular spirit will have only one materialized form—it will not change forms). Considering that literally hundreds of magical traditions are practiced worldwide, the sky’s the limit. If you’re stuck coming up with a snazzy and distinctive look to represent a particular spirit in your game, here are some examples:
That should be clear as well: materialized spirits are immediately visible as such entities under normal conditions.
Now, lets go to the Realistic Form power
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Realistic Form
Type: P • Action: Auto • Range: Self • Duration: Special
A spirit with Realistic Form can be mistaken for a normal physical creature or object when it materializes, or it appears unremarkable when joined to a vessel. A spirit that appears as a metahuman would have a heartbeat and a regular breathing rate. A spirit that appeared as an object mimics the object’s normal functionality; for example, a toaster could be plugged into the wall to toast bread (though it would have no Matrix link, making it an antique toaster). Th e spirit is in no way disguised from the astral plane, but to physical observation appears to any senses to be a natural part of the physical world. Note that spirits with the Materialization power normally only have one materialized form. Materializing spirits with this power can choose to appear using Realistic Form or their normal materialized form. A fi re elemental can still appear as a column of angry f ames, but might also be able to appear as a beautiful woman.
Now it depends on how we define "mimics. Lets take Falconers route for second: the sword spirit looses the physical ability to move. It receives the ability to do sword damage with reach 1. Out of nowhere you take away the the physical ability to move, by defining that the spirit is *limited* to the function of his form. Ok, so a sword cannot move. Can it rust? What about "Immunity to normal weapons"? Because that is a physical power as well, depending on the materialized form of the spirit, exactly as the 3D movement.
So, following your own argumentation the spirit looses the immunity to normal weapons because immunity to normal weapons is definitely not a part of the normal functionality of a sword (after all, itīs just a piece of sharp metal, nothing more).
Lets go back to the first rule paragraph: a spirit in human form does breath and have a heartbeat. As per your definition the spirit is limited by the physical traits of the body. Can it suffocate (after all, suffocation is part of the normal human body if no breathable air is available). What about bloodloss? And of our Immunity/weapons (after all, no human is bulletproof). Is it hungry? Can it die/be disrupted by not eating (after all, eating is one of the most basic functionality of the human body). Would a spirit car be able to run 200mpoh on its own (without movement power). Could a van-spirit transport 2000kg? Can a female human-spirit become pregnant (which is the most basic function of the human body beside sleep and eating)?
If you go this route, please be consequent and accept ALL physical functionality and limitations of the realistic form of a materialized spirit. Not simply: "you loose movement, but everything else stays the same". That would be inconsequential.
For my part, as per rules: the materialized spirit is exactly that: a materialized spirit, easily visible as such, with all the powers. If he has the realistic form power, he appears are real - but is still in every way a materialized spirit with all advantages and disadvantages (in our case: no specific form depending damage but the general spirit damage in close combat, immunity/weapons, 3d movement etc). There is NO single sentence, that the rules stated above are invalid when the realistic form is activated. Only that you get additional abilities to stay "undercover" with performing minor tricks.
And still of course: thanks for all the points discussed above.
SYL