I'm going to have to be the wet blanket here and say I'm against this. Blocktopia has always had a strict policy against donor benefits. Any incentive like this, even if it isn't game-breaking, rewards donation with items, and I believe this goes against Blocktopia's standards. Even useless items could potentially make poorer users feel lesser due to their lack of irl money. I love the ingenuity of the idea, but I don't think it's right for Blocktopia.
Also, I don't intend to debate my opinion. Along with many others, I'm sick of arguing. I just came here to pose my own idea and opinion. If I've misunderstood something, I apologize! I just feel as if it's important to get both side's perspective.
EDIT:
Da Jinks has brought up an excellent point about the ability to trade valuables for keys. If we were able to destroy the crates and not have to carry them around, many of us would be for it.