You don't really need one, unless you are really worried about scratches on your case. Mine didnt' come with one, and they sent it back on my door step in less than 96 hours, looking better than when I shipped it off with Gateway's box.
Their service seems to have gotten a lot better, the turn around time was just amazing and the job seems to be well done too. At least so far. The plastic of the back panel of the LCD seems to be made from a different type of plastic, with less flex. The strength of the casing could have been one reason that all the cracks started forming. Only time will tell though.