I'm not sure how to read the rules for Parma Magica. Page 85 says
While a magus is protecting others, his effective Parma Magica score is reduced by 3 points, both for himself and for the other people he is protecting. If the magus has a Parma Magica score of 3 or lower, his Parma Magica provides each character including himself, with a Magic Resistance of 0.
Should I take this to mean that if his score is reduced to 3 or lower (say, from 5 or 6) his Parma Magica provides each character including himself, with a Magic Resistance of 0? Or if his original score is 3 (or lower and then reduced by 3 to 0) his Parma Magica provides each character including himself, with a Magic Resistance of 0?
The rules then say
In the magus's case, this is added to his Form resistance, and a magus protected by another magus's Parma Magica may also add *his Form resistance.
(*his meaning the magus casting Parma Magica or the one being aided by it?)
If the adjusted score is 0, and all magi have Parma Magica of at least 1 why would a magus aid another magus, if the one being helped is better off casting it on himself instead?
Clearly I'm confused. This is what I love about trying to learn a new game system at 4 o'clock in the morning.
I believe it uses your 2nd example. Mages with parma magica of 3 or less, can protect a group of people with a resistance of 0.
A mage would add his own form resistance, not that of the mage protecting him. (I think, it's an interesting question)
A mage may desire another mages protection if his parma magica is much better, 4 or more better.
Or, if he's a non-hermetic mage, and doesn't know the parma magica.
Or if he's still an apprentice, and hasn't been taught it yet.
It says that the effective score is reduced by three; BUT if that reduces it to 0 or below, it still counts as "MR 0".
The possible IC reasons a mage might want the protection of another mage are varied and obscure. I believe the word "this" refers to the entire process, not specifically limited to the situation if the initial Parma score is 3 or less. The point is that any mage so protected uses their own Arts as bonus, not the protecting magi's.
That line is there to answer predictable questions.
(Three reasons that jump immediately to mind are... the protecting mage has a MUCH bigger Parma; the protected mage is unable to perform the ceremony (unconscious, or as TB suggested above, or...?); or some obscure IC social restriction - perhaps a lost Certamen bet.)