Given the strength and scratch resistance of Gorilla Glass, are cases of value for iPhones?
I have an iPhone 4S.  Like you, I originally thought that I ought to get a protective case for my iPhone.  I tried several cases, but none of them were satisfactory (too bulky, too cumbersome to open to get to the phone, too ugly).

I now have no case for the phone, and I keep it in my trouser pocket all the time.  Instead of getting a case for the phone, though, I got a key case for my keys (which I keep in the same pocket) to prevent them scratching the glass!  That solution has worked well - the iPhone is now over a year old but looks as good as new, and I haven't added any size or weight to it.