The thin phone with special finish that "needs" a case is still thinner than thicker phone with finish that needs case just as much (or little). Thickness is a red herring here.
Alternatively, by making it thin and small, they've enabled customers to still have a reasonably sized phone after they've customized it with their choice of case.
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If the phone has a nice showy fragile finish users may feel compelled to protect it by putting it in a case. Negating the benefit of a thinner phone.
The thin phone with special finish that "needs" a case is still thinner than thicker phone with finish that needs case just as much (or little). Thickness is a red herring here.
Alternatively, by making it thin and small, they've enabled customers to still have a reasonably sized phone after they've customized it with their choice of case.