One thing (as mentioned) is fade. Remember the big fad of buying 'bras' for the front of your car? They protected from road debris causing chips, protected the finish, etc...The problem later, removing the bra and finding that your car was now a two tone model. The area protected by the bra, No fade, everywhere else, faded (even white cars can change slightly).
A partial cover that comes down only 1/3 the way can cause the same effect (depending on your location and sun intensity - less up north, more down south).
Just some more fuel for the debate...carry on. ;)