Using magical defenses against 'cosmetic' spell effects

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 Aegis does 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 Aegis does 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 Aegis does need to Penetrate the Magic Resistance of the caster.

This seems consistent with the rest of the spells.

That drastically reduces the worth of the aegis...

Maybe, but that is how I think it should operate, if you want it to be consistent with the rest of the rules.

Whether it should be consistent with the rest of RAW on Penetration is another matter.

Heh, just another reason to ignore the RAW on wards....

Cheers,
Xavi