My argument is that the iPhone shouldn't need a case to be safe. It's not printed on the box or on the website "recommended to be put in a case". That's total BS if they deny you. I always roll with my iPhone naked because cases do more to cheapen the device and look retarded. If iPhones need a case, one should be included in the box.
I hear a lot of similar arguments to this one: "If the phone needed it, the phone would have come with it."
The moment you purchase the phone, the physical, tangible equipment becomes your property. The responsibility is yours to protect the phone and take care of it. If you drop the phone and it breaks, it's not Apple's fault. And I don't recall any mobile phone that states that it is recommended that you keep the phone in a case, but there are many that would benefit from being put into one;the iPhone, especially.
The iPhone is not the most durable of phones. It was not meant to be. It was meant to be a good looking phone that could handle minor rough housing. If you want to just roll naked, that's fine. I've met more than just a few people who do the same and who've dropped their phones and cracked it to all hell. If those same people kept the phone in some sort of case, even a thin slip in case like my SENA, the phone would have been fine.
We all have the freedom to choose whether to keep the phone in a case or not. But I don't care how careful you are, there is always the possibility that you'll drop your phone on something other than a soft surface one day. And when that day happens, you're going to wish it was in some sort of protective casing.
BTW, Apple may have been asking you if you use a case in order to develop a census. I'm sure they've been getting back a lot of cracked phones. Just because the cracked housing is well documented in forums and a few online articles, does not mean that it's wide spread knowledge outside of the Internet. If you brought me a phone that was cracked, I would ask you the same question: did you use a case?