Remember MAGs from Phantasy Star Online?
It might be interesting to see a similar function in SK. Feed your little buddy your spare materials and help it grow up into various forms that can provide you bonuses on deeper tiers.
This would also influence Auction House prices, as people would be spending their materials on their MAGs instead of creating as many auctions. Similarly, crafted items wouldn't appear in the AH as often, as players wouldn't be crafting using those spare materials as much.
Alternately, the developers could hit two snipes with one stone and tie this in to Guild Hall upgrades (which a lot of players seem very eager to get). Instead of individual MAGs/pets, guilds would get a "material deposit box" in their guild hall and deposit their spare materials in much the same way we do with minerals during gate construction. After a certain (hidden) number of materials have been deposited, the Guild Hall undergoes an upgrade process and builds itself a new wing, merchant, view, bulletin board, or whatever. The upgrades might be fixed (obtained in order or by certain material "recipes") or random (to encourage more experimentation when depositing materials).
I think these would make great material sinks if that is your intent.