QUOTE (SR4, page 289)
Materialization
Type:P * Action: Complex * Range: Self * Duration: Sustained
Certain astral critters are capable of projecting themselves into the material world, thus allowing them to interact with physical beings. When materialized, critters may affect physical targets. Additionally, materialized critters gain Immunity to Normal Weapons.
Type:P * Action: Complex * Range: Self * Duration: Sustained
Certain astral critters are capable of projecting themselves into the material world, thus allowing them to interact with physical beings. When materialized, critters may affect physical targets. Additionally, materialized critters gain Immunity to Normal Weapons.
That looks fairly unassuming, yes?
But wait, look at this:
QUOTE (SR4, page 286)
Type: Powers may either be mana (M) or physical (P), just like spells (see p. 195). Mana powers do not affect nonliving targets, whereas physical powers cannot be used in astral space or affect astral forms.
Did you catch that?
Let's go over that again. Physical spirit powers cannot be used in astral space, nor may they affect astral forms. Spirits, before materializing, are astral forms. Materialization (and, coincidentally, Posession and Inhabitation) is a physical ability.
Conclusion, based on RAW: Spirits may not use the Materialization, Posession, or Inhabitation powers, and are therefore completely and utterly unable to render aid on the material plane, and are nearly completely useless as a result.
[ Spoiler ]