I use the Apple Leather case because it is a thin footprint that is minimalistic in design. It also adds a grip to the slippery feel of the phone so it makes it easier to handle.
I never use cases that add bulk or anything. Before the Plus I used transparent bumpers to help with grip because, while I'm not a phone dropper, the grips certainly made it harder for me to make any mistakes with it in my hand.
I also resell the phones every year (or two), so adding a little protection increases its market resale value to a mint condition phone.