MVS kits go for crazy money and so if you have a mint kit or paid out, I see no problem with having a protective case to ensure it keeps mint).
It's not like it's a sealed case and you can't play the game.
It's no different to the Windy sleeves imo, which I think are a great idea to keep your carts clean/dust free etc
Don't hate, corrugate.
You can find it here
http://www.saveyourgames.it/index.php?route=product/category&path=35
Even if you own an MVS kit box protector, do you have an MVS kit box protector protector?