If I got a used game in a box, I'd keep it. My friends with Nintendo 64s (some of whom have since gotten rid of them) used to keep their un-boxed games thrown everywhere in their entertainment centers.
Not only did it look sloppy but what if they stepped on one of those things? I've heard of guys tripping on golf balls left on fields and being sidelined for months as a result, so I'd hate to see this happen to someone leaving a game somewhere. Also, they would have nowhere to store their manual and if they step on one of those, I'd be worried about breakage.
Of course, I'm one of these "keep the original box" guys. I have the box for my computer and keep my old one in its box (I haven't used it in years). Heck, I keep my artificial Christmas tree in the original box.