Idea on banking with oaknots

So, basically the idea is to let the game's certain NPC buy the materials you buy with oaknots and pay you with the same amount of oaknots. It sounds weird but here's why I'm suggesting this. If you're keeping these materials in your retainers and your inventory for later use, you can now just keep them in the npc 'bank' and have it take up much much less space in your inventory. It also comes with the fact that you can then take out different materials that you banked though (put in material A to get oaknots, next time, you can withdraw material B with your oaknot). This system is also probably restricted by the fact that you can only get the same tier of materials and items due to their economic worth. However it should still cut down on inventory usage and helps out both players and SE, since they're apparently having trouble giving players more inventory because of 'save data' issues.

Anyone care to give input on why this is a bad idea? How it's going to affect the economy (since I don't really deal with them much), how effective do you think this is going to be with just the current oaknots and the materials it is used, whether having multiple types of oaknots for multiple tiers of items is feasible, whether having said multi type currencies is any better than the one way oaknot or previous philo/current myth currencies that are only kept track on the character tab, should only materials be able to be 'banked', etc.

Can't I just sell materials I don't need on the marketboard and buy the stuff I need from it?
The intermediate currency could have a simple name - like gil.

Sorry to say, but aside from the retainers we don't need an additional bank (except an accountwide one)