I don't know if this differs from country to country, but the vast majority of iPhones I observe are not in a case — probably at least 90%. The design is a major selling point, so I'm not surprised most people don't want to use a case.
Most iPhones I see around are in a case. I personally don't like that, but if you're going to type on a flat table, you'd better use a case to flatten the back of the iPhone, avoiding those rattlings.
> My 6s has a scuffed camera ring, and rattles when used on a table, due to its inability to lay flat.
Exactly. This was my first impression when I saw that protruded camera on iPhone 6.