From Fallen London Wiki

Even overpriced, it seems to be the best torso for spider popes. Or at least better than segmented ribcages, which take at least one more action to make into a spider (no tail slot on carapace).

Leaving off the value of the legs (cancelled out of the equation), the value of the skeletons plus theology is: Segmented - 67.50 (60 from theology, 7.50 torso/head/tail) Carapace - 70 (60 from torso + 100 from theology - 90 from being overpriced)

During spider weeks this is more pronounced as you are boosting 60 instead of 7.50.

And since it takes fewer actions to fill up a carapace, better EPA.