There's a popular idea that the reason Apple uses such minimalistic design that emphasizes thinness rather than coating it in rubber or colors is because they only make the "core" of the product and leave all the customizing to the consumer.
If you want an easy to grip purple phone then you buy a purple TPU case. If you want an overly fancy rich girl phone then you buy a pink designer case. If you want ultra durability then you get a hard case. If you want your phone to look like a bear you get some weird incredibly awkward to hold bear shaped case. And if you want it to look just like it does in the commercial then you carry it without a case.
This "core" idea makes the iPhone the most personalized phone available because with such high popularity, minimalistic design and two available colors then its pretty easy to make such a popular device look different from everyone else's phone IF you want it to.
Personally, I prefer no case but with the iPhone 5 then I think there are many reasons to use a case on these:
- Hide manufacturing blemishes
- Protect the delicate aluminum from dings and scratches
- Maintain condition for resale value
- Comfort from hard edges (IMO, this is Apples most uncomfortable phone due to hard edges)
- Add girth/weight to a phone that may be considered too thin or too light