Personally, I ONLY use original Genesis cases for these games. For me, I am lucky enough to have some used game stores that sell some sports titles pretty cheap complete - around $1 - $1.50 each. I buy 10-15 of these at a time (I'd get more, but on the off chance someone actually WANTS Joe Montana Sports Talk Football 94, I leave a handful) Then I just strip and refill. I used to save the labels and manuals...but now they just get tossed. Carts are saved for donors or sent to the local goodwill (the one I DON'T go to, so I don't have to worry about getting my hopes up when I walk in only to find that pile of Genesis games is the same crap pile I gave them).
As for WHY...I think I'm just a Genesis fan, and since those boxes are common, cheap and easy to acquire, it's a nice change of pace from the UGCs that house all the other carts on my shelf.