The current situation for thos levelling multiple crafting classes is that you have to go back and forth running to the summoning bell and shops to get any craft mats' you have, especially when switching around from class to class.

Seeing as how there is no major use for crafting mats in battle, it would make things extremely convenient to let me use crafting mats out of my retainers' inventory directly, without needing them to be in my inventory.

Demon Souls had something similar; where going to a blacksmit to upgrade your weapon you didn't need the materials to be in your inventory, you could've used them when they were with the retainer.