I might sound silly, but for me, the case isn't even really about protecting the phone. For any phone I own I usually have a collection of several cases that I swap between depending on my mood. It's a way to customize the phone and make it look and feel different. To me, a new case can make a phone feel like a brand new device which can actually help me scratch that "new toy" itch when I know that I really don't need to upgrade.
Also, I don't really like the way current iPhones feel "naked". Many cases make it feel much better in the hand, in my opinion. I definitely favour thin, sleek, "minimalist" cases though, as I dislike adding too much extra bulk/thickness.