For me, the rounded, slippery metal backs in the thin frame of modern iPhones would prevent me from truly enjoying one without a case. Partially because of the drop risk, but also because it is simply a bit stressful (not sure about the right wording here, non-native speaker) to keep a firm grip and use it naturally.
The design is less important, as I usually look at it from the front side (a screen is a screen, no matter if a case is on or not) and don't want/need to show off anyway.
Let's put it in a different perspective: The modern, slim design allows to add a case to it without mutating it into some ugly behemoth.