Taking XI as an example, some sets were great, some had a couple good pieces, others were pretty terrible.

For BLM for example, you can only get INT on legs, head, and weapons via materia. Other slots have little-to-nothing (except some ok bodies). Bonuses unique to the slot are enough to warrant use over materia. VIT on PLD head, heal potency on WHM body are other possibilities. Unique effects and set bonuses wouldn't be bad either, but unique-to-slot seems more likely.