If Gren wants anything more than hollow point or armour piercing rounds, why not buy it off an in-RP shop? Why not be able to tangibly resupply? You say a 'good explanation' of how he got all those toys, but that doesn't matter, only the Credits should explain that. We have CREDITS for that part.

So therefore, why do we even get lien as a mission reward? Spending habits? Please, every student's spending habits depend on their familial backgrounds too. For instance Weiss is a rich girl and spends as much as she wants, though rarely in RWBY are characters seen going shopping like that.

Basically, we have no tangible use for lien, and therefore it is pointless at the moment.