The problem with buying cubimal boxes is that, fun as it may be, one always ends up with multiple common cubimals. My idea is some sort of 'trade-in' function, where a number of common cubimals could be traded in -- say 5 or 10 at a time -- for one new cubimal box.
This isn't a 'free' box as you have to spend a lot of currants to actually get it (unless you find common cubimals out and about in Ur), and there's always a chance the new cubimal box may contain yet another common (in which case, you're in no better position than when you started). A use for common cubimals would also push their prices up in the marketplace somewhat (well, it'd be above 'free', at least), which may be a pro or con depending on your point of view.
(I know there's always the option of shoving all those commons into SDBs, but I see them as little cuboid reminders of all the currants I've burned looking for rares...)