One could also ask the question "how many people would prefer to have a phone that didn't --need-- a case"?
Given the choice between a thin phone that needs a case and a phone that is as thick as a thin phone with a case that is more durable, possibly has a headphone jack and a slightly larger battery, I would personally go for the latter.
If a phone is designed so the screen always breaks when it hits the ground unless it's protected by a case, it would make more sense to manufacture the phone inside a case already, no? I.e. make a raised rubber bezel around the screen's edge part of the standard design.
Given the choice between a thin phone that needs a case and a phone that is as thick as a thin phone with a case that is more durable, possibly has a headphone jack and a slightly larger battery, I would personally go for the latter.