If anything is ridiculous, it's the existence of this topic.

For starters: Any content where there is no white mage present, mainly due to 4-man content, has no significant magic damage flying around anywhere.
Additionally: If there is continuous magic damage present, Eos' Fey Covenant actually offers more magical defense than Shell does for the moment you need it.
Lastly: They're dungeons, content that has nearly no difficulty value. If you desperately -need- that magical defense effect on the tank to be able to make healing work, I simply don't know what to say.

The list I mentioned may be ridiculous, but it's in line with your way of thinking: If there's no scholar for dungeons