OK, the Aegis needs to penetrate. What effects will this have?
It's explicitly stated (HoH: TL IIRC), that we can vary the level of the Aegis spell, but not it's parameters without a breakthrough.
Thus they are all similar spells. Now ever trouble maker in the Order is going to pick up an Aegis spell (level is irrelevant, 5 is fine) and master it with resistance mastery. Double your MR, unlikely to be cast with an penetration multiplier specifically to you => irrelevant spell.
I'm aware that with penetration multipliers MR is useless, but without them Parma can be very effective.
But let's take it a step further: What's it penetrating against?
If we're talking about some silly demon trying to sneak into the covenant it's not very complicated.
Unless ofcourse said demon is possesing someone that should be resisted, but that's another story.
Let's instead look at a spell. Spells have no MR, so obviously the spell itself is not what people say the Aegis needs to penetrate.
If magus Tellus tries to teleport himself into the grounds of another covenant, presumably it needs to penetrate his resistance?
Now, Tellus is a wiley old fox, so instead he uses a touch range teleportation spell to send his sodalis Dophus.
They are both present at Tellus' covenant, with it's own Aegis.
Now Tellus sends off Dophus, to a covenant where neither of them have a casting token. Who's to be penetrated by the Aegis?
Dophus, who's moving into the area protected by the Aegis?
Tellus, who's casting the spell?
Presumably it's Tellus, since otherwise the Aegis would be useless against scrying spells.
Maybe Dophus as well, I don't know?
But Tellus is protected by his own Aegis as he's still at home.
So the foreign Aegis would have to penetrate that Aegis to affect Tellus, correct?
So Tellus Aegis would need to penetrate... someone, to protect Tellus from the Aegis into which he's trying to send Dophus...
Who would need to be penetrated by Tellus' Aegis? The original caster? Who's presumably a group, casting with Wizard's Communion?