Never. Cases are ridiculous. No one cares if your phone has a scuff mark or scratch. No one. Nor does it matter, ever.
"Oh, what about resale value." The money you theoretically gain from having a "flawless" iPhone just went out the window when you bought the case. Further, the extra $50 you get three years from now for having less marks -- unquestionably not worth lugging around some stupid case.
If the iPhone can't stand up to everyday wear and tear without a condom, I don't want it.
All cell phone cases look stupid. All. So do those ridiculous plastic "screen protectors," I've never seen a bigger waste of time or money, they don't protect a damn thing. I've never used any of it, and my phone works perfectly fine.