I would say the Aegis sometimes needs to Penetrate.
To protect against incoming effects (i.e. incoming effects passing through the Aegis), I would say that the Aegisdoes not need to Penetrate.
To protect against incoming critters (i.e the Aegis acts as a ward against creatures of less than Level Might), I would say that the Aegisdoes need to Penetrate the Magic Resistance of those creatures.
To reduce the Casting Total of tokenless casters who are standing within the Aegis, I would say that the Aegisdoes need to Penetrate the Magic Resistance of the caster.
This seems consistent with the rest of the spells.