I guess my point is that I'd like to see a study like this, and I'd like people to stop saying "My case has saved my phone so many times," because nobody really knows that.
I'd like to know how much and in what situations a case actually is likely to save your phone from meaningful damage. Because, to use the analogy, my phone's been in a bunch of car accidents without its seatbelts -- non-trivial ones -- and is completely fine.
It would be one thing if a case didn't come with the added cost of a bulkier phone (as you mentioned, rubber cases make the phone fatter). But they do, so it's a cost-benefit analysis, and I think we're all mostly guessing. My assertion is just that I think most people are guessing too much on the side in favor of having a case, because iPhones are not as fragile as they seem as long as you treat them with a reasonable amount of care.