If the battery is removable, like most android phones, you don't need the protective case. If dropped, the battery will come out and take a large force of the fall. iPhones do not do this and need the case. Then it became either trendy, needed for resale, or people don't know that their phone 'exploding' when dropped is a feature. For example, I have dropped my EVO V at least 20 times, onto concrete or tile, without issue.
If the Google Nexus 4 is meant to show the direction Google wants to take Android phones in, "most" android phones will not be robust for long. While the newest iPhone has done away with the glass back, the newest flagship phone from Google has a glass front and back and a non-removable battery.
