Honestly, as I've gotten older, I've grown to dislike loose cartridges. I still don't have a good way to display them, and I ultimately just feel better about my games having a box as protection, and the box in it's own protection.
I've mostly stopped buying or trading for loose carts (always hated disc only). Obviously, if I find some dirt cheap in the wild I will make an exception, or if the game is of extreme rarity to where a CIB copy would cost a kidney & and a half...
Otherwise, I want my games nice and boxed so I can enjoy them from afar and get that little bit of nostalgia opening the box, flipping through the manual, and replaying a classic!
As for boxed without manual, I'm usually cool with that too. A boxed copy missing the manual will still ear a spot on my shelf. Now my empty cases I hope to one day find carts/discs for... those I refuse to put with my other games, and are secluded to their own shelf...
Besides the Mario Kart Wii promo case, I'm hoping I can complete some of the boxes on that shelf, and move them over with the actual games!