well, it's cheaper and you got protective films for front and back unlike the bumper.
It doesn't matter how you case up your iPhone 4, it'll always look similar to an old iPhone because it is the front face that shows in every case and funnily enough, the front face between all of the iPhones look pretty darn similar.
This is why I want the iLuv case over the bumper, it retains the square edge. the bumper makes it round, just like the old iPhones.