I think it would make more sense if leather were moved to Quest Items instead of the Materials tab. It's only useable during quests anyway and that would free up an inventory slot for the people who don't destroy or otherwise get rid of the extra leather between quests.

Quest tab is for items that most likely will be never used after a particular event ends. This way players can keep collectibles that are useless other way. Items that are used from time to time are considered to be regular materials. It's not a collectible but an item that have some type of use. The same concerns rare jewels, metals and pigments.