People act like a case is magic shield that will somehow stop the forces of physics and reality from applying to the enclosed device. It's simply not true. Here's some of the issues cases cause:
•Apple designs their devices to cool through the body. Adding a case is adding insulation. Like you putting on a parka when you really want to keep cool on your run.
•retaining and concentrating heat in the body can, and likely will, lead to different rates of expansion in the metal, possibly causing a permanent bend.
•most cases snap on the enclosed devices and likely cause a compressing force that over time can encourage bending and warping unless the case is very, very well designed.
•different metals/alloys have different levels of 'memory' that may allow them to return to their original shape when they cool again, some do it very well some nearly not at all.
•A case is just about guaranteed to collect dust and debris between itself and the device causing micro-scratches in the surface. This happened a LOT with the colorful iPods with the anodized color cases,
I'm hard pressed to come up with any other high-cost thing in our lives that we purchase then put in a case. Not our TVs, stereos, refrigerators, cars, jewelry, etc. You'd laugh if you saw a Rolex or a gold ring in a case. Who sold us this bill of goods that a case is needed for our mobile electronics?
Fact is that if you put enough G-force through a sheet of glass it will break, enough torque on a plane it will bend, enough point pressure of metal it will dent. A case may make it harder for those levels or force to get to the device but why don't you keep it out of the case and just treat it with the care and respect that an expensive device deserves?