Earlier quoted context omitted.
It's not about preserving it in mint condition, it's about preserving it in usable condition. I can't speak for others but my case has saved my phone from a smashed screen on several occasions. EDIT: Should possibly also add that I have small children (1 and 3 - that's their ages rather than their names) who use it and I'm damned if I'm giving it to them without a case on...
How do you know that the case saved the day? I once had my case-less iPad fall from a ledge five feet above the surface it eventually landed on and it survived with nary a scratch. They seem fairly rugged on their own.
I think the cases were really important, but once the HTC Hero and that generation came out, then magically my phone lasted years!