As a person who insisted to always use all her iPhones - starting from the iPhone 2G up to the iPhone 6 Plus - without a case and without ever breaking a single screen, I'd like to share my opinion...
My iPhone 6s Plus is the first phone I put a case on (the original Apple silicone case) from the first moment I took it out of its box.
I decided to get the case, because I remembered the fact that on my previous Plus, the back became even more slippery than it originally was, throughout the months of using it, because of the oil on my skin. I surely gotten used to being EXTREMELY cautious and alert at all times -and I mean ALL times - but it was the first time I felt quite bothered by this.
When I first bought the 6s Plus and put the case on it, I was not positive at all. I really hated the added bulk and that I could not feel the curved edges of my device.
But at the same time, I felt relieved, because I suddenly didn't have to be so careful with my phone, for the first time in 8 years!
After a while, I got extremely used to having the case on the phone and I did become more careless, because I didn't have to take so much care of the phone anymore, the case did the job for me!
I have already dropped it once, and had to replace the case because it had a huge bump in one of its corners. I won't say that if I did not have the case, my screen would now be cracked, because as I said, I stopped being so paranoid and careful, now that I have the case on.
So, now that it's been two months that I've had this phone with a case on it, I believe that the extra security that a case adds is much, much more important than a phone's aesthetics, for people who are really paranoid about keeping their phones in pristine condition.
I'm sure I will never go without a case anymore and I never thought I'd say this!