While the aluminum body of the MacBook Air/Pro might be nice to look at, there are some who might not enjoy the touch of cold metal on their palms especially while typing. That and there are those who are hoping to avoid staining or scratching the body of their MacBooks which is why you might see MacBooks with a cover or a palm rest. It sounds like a good idea and you think that this will help in protecting your laptop in the long run, or possibly because you think that the palm rest you chose looks really pretty, but whatever your reason, it seems that Apple is advising users not to do that.
According to Apple’s support page (source link below), the Cupertino company has warned against the use of palm rest covers as it could damage the display of your MacBook when you close it, stating that, “To enable thin design, the clearance between the display and the palm rest area is engineered to tight tolerances. Do not use palm rest covers as the added thickness may interfere with the designed resting position of the display.” This isn’t to say that you can’t use palm rests while your MacBook is open, but it could be pretty troublesome having to take it on and off, especially if your palm rest is stuck on.