It seems the back is more scratch prone than the Gorilla Glass front. At the same time, the back is decorative, so scratches won't affect function, and different people have different views on whether they "need" to keep their tools looking pretty, or just working right.
Even among people who want to keep a phone looking good, the need for protective films or skins is going to be different for someone who uses a phone holster than for someone who throws their phone in their pocket (or purse) with keys and a pocket knife and sand.