As outline in the PFS guide, players are not allowed to use their prestige to purchase spellcasting for other players:
Prestige Points are designed to be spent by characters on themselves.
Does this apply to rewards that affect multiple players? For example, can a player purchase Break Enchantment (which has a CL of 9) and include other players in the target? It seems a pretty big waste to only allow the player to target himself.
|⦵ Lau Bannenberg Venture-Lieutenant, Netherlands—Leiden aka Ascalaphus|
|Wei Ji the Learner|
|⦵⦵ Sebastian Hirsch Venture-Captain, Germany—Bavaria|
There was actually a precedent for this sort of set in Cosmic Captive, as players could elect to spend prestige on a group buff...
The PP option is cheaper than the spellcasting cost and you are likely to far away from a city at that point to get spellcasting service for the normal price.
In any case, yes PP are designed to benefit the player who earned them, but you can already spend them on your animal companion, and there are situations where spending them on another character would directly benefit you.
So my answer is pretty close to Lau's I don't really see the downside, but on the same token players should not be/feel forced to spend their PP to helps other players.