AFAIK, dirt and feces are rich in cobalamine (vitamine B₁₂), which is otherwise scarcely available in non-animal sources.

Calling it a "disorder" and "largely non-nutritive" seems a bit broad, when it could be satisfying a particular need. Even though eating feces might not be a good idea and more serious examples do seem useless and damaging.

Surely you'd have heard about it from friends or family if it was common for pregnant women to have cravings for nails and bottle caps and that their bodies need the nutrition from them?