I keep them. Maybe one day I will throw them away....probably not.
Yeah it'd be silly to toss, them. They're high quality (usually) plastic cases.
I like the battery storage idea above. I bought a couple of Sanyo/Panasonic Eneloop rechargeable battery kits. They came in nice little cases. I put all batteries in one case (each had empty slots). I keep both cases in the kitchen catch all drawer.
One serves as the toolbox for the house tools -- tape measure, small level, screwdrivers, pliers, electrical tape, etc.. Other has the batteries.
I had a couple of Makita tool cases (rigid plastic). I gave one to a friend with some basic household tools in it. The other one holds my loose tools and materials (clamps, fasteners) and I keep it with my portable toolbox.