I used to put cases on my phones, but now I just use a skin on the back and screen protector on the front. This is because (1) Looks way better than a bulky case (even the slimmer ones add a little bulk), (2) Doesn't take away from the phone's design, (3) Most of the time cheaper than a good quality case.
Sure, a case most likely will do a better job at minimising scratches, dents and other stuff that might damage your phone. Personally, I sacrifice this reassurance and get a grippy dbrand skin on the back to protect from micro-scratchs and a screen protector on the front because my phone doesn't have an oleophobic coating on the screen.
Plus, I'd like to think I take good care of my phone . Just don't drop it.