Parma is one of the Order's breakthrough. Few non-hermetics can muster the same Penetration totals as hermetic magi can. Within the Order, it offers decent security (and cooperation!) benefits (especially if combined with an Aegis) even if it is not a perfect defense.
On a metagame level, it makes things more interesting if the Order's club is stronger than the Order's shield, precisely because the Order can then bash itself on the head. Politics. And yet it is not as Order-breaking as the identically-named breakthrough (in HoH:TL?) which ignores Parma.
The spell itself is interesting because, being its own countermeasure, once it spreads, you cannot afford not to know it. Yet, seeking it is suspect - unless, say, you're a renowned hoplite. Its only use is against the Order... or to protect you from members of the Order - including Quaesitores.
What are you afraid of, sodalis? Do you not trust your sodales?
Consider even a book on spell mastery. They're still suspect! Are you going to use it to learn Penetration Mastery or Resistance Mastery?
In the end, I wouldn't mind if one of my players introduced the spell. But then again, they're already in enough trouble that they would be leery of anything that would alienate the Order any further. Though it would be a fun spell to slip into their library to make them sweat. It wouldn't be the first landmine they have found in there.
Your saga may vary.
Here is another Hermetic-bashing spell for your perusal. Much, much more limited in its impact, and just as straightforward an application of the guidelines, but still amusing. Using it is explicitly a High Crime.
PeVi 20 - Blunt the Spear of Hermes
R: Voice D:Sun T:Individual
Reduce all Hermetic Magic casting totals of the target by 10. Note that this is done after division for spontaneous spells.
(Base 3, +2 Voice, +2 Sun)
I once toyed with a Gauntlet-level PeVi Bjornaer magus build with a short-range teleportation ability ("Leap of the Stealthy Cat") from Mythic Blood, and Touch/Diameter/Individual Hermetic-bashing spells (Parma blasting, casting total-blasting...). Can't remember how I formulated his Minor Focus, though, nor if I relied on virtues or mastery to get silent/still casting.
As a Hermetic-hunting one-trick pony, it was terrifying, even without bothering with any Mysteries.