Before I switched to Android, I had a Lumia 920 for a year. Never had a case for the phone. When I bought my HTC One M8, it didn't have a blemish on it (the Lumia). Before the Lumia, I had an iPhone 4. Again, no case was used. After a year, I sold it to Gazelle in pristine condition. That being said:
I know I'm flirting with disaster. I'm very careful with my phone. I always place it in my front right pocket, with no other items inside. Also, after the iPhone 4, I decided that I would not buy flimsy, fragile phones. I feel that a case covers what are now, works of art. It adds bulk that diminishes one of the reasons you bought the phone in the first place. That being said, I do realize that there are people that must use a case. Whether it is the environment the phone is exposed to, or the owner being clumsy. I never criticize those who have a case...It's just not for me.