Why exactly? In my experience, at least half the people I see don't use a case.
And for those that do, most would be simple rubber or leather protectors with no reinforcement, just to keep it shiny. They aren't going to stop a phone bending if it is prone to bend.
The benefit that applies to the largest swath of people and cases is adding a slight ridge around the screen so that it's less prone to scratching on drops. This is, and has been, one of the things iPhones are more prone to than some others because of the 'slick' design of having very flat faces. Then there are drops themselves, and even rubber bumpers help there especially with phones that are mostly metal and glass. Yes, a silicon sleeve isn't going to help with the bending and a plastic shell isn't going to do much if anything either for that problem. My observation was mostly that none of the phones shown with issues had cases sitting next to them, or screen shields, or any of marks left from cases. Yes, it could be that they took off all that and cleaned to phones for the pictures...which to me seems and odd choice when showing damage done to a phone. One would think that protective measures, even ones that wouldn't help with that specific issue, would be left to show that they weren't unaware of protecting to phone and that it was a real, used, unit and not bent just to show they can bend.
My experience selling phones was that case use was around 70-80%. More importantly, phones that had issues were far more likely to be ones
without cases. Now that might be correlation in that people who get cases are more likely to be people who are concerned about taking care of the phone in the first place, but there were enough issues that would/could have be prevented by a case and screen protector. Screen scratches effecting usability were the most common. But even good screen protectors are dirt cheap. If you're worried about putting the protector on without getting a bunch of stuff stuck to it, ask at your provider for the person who's good at putting them on to clean your phone and put one on for you. That was me at my old job, and although it could take some time (especially with the lower quality screen protectors which were NOT always the same as the cheap ones) I would almost always get them on cleanly, with at most one small hair in the non-screen area. The two times I messed up and got something stuck to the protector I couldn't remove with spray, we member sat'ed a new one and I put that one on.
Back to the main issue, I'd advise a fairly strong case for people new to large phones. I was going to get an Armor series Otterbox for my Note 3 (so I could keep it on me while doing construction and getting wet), but it never came out for that phone. In fact, I'm still rocking the little cheap leather and plastic stand/wallet/case I got as a placeholder. I just put it in a plastic hardcase when I'm doing heavy stuff, and in a Ziplock freezer bag when I'm hiking or doing something where I might get wet (and sometimes to read in the bathtub). But I'm large guy who often is carrying some kind of bag. My pockets are large enough to hold the phone without bending even when I sit. I don't have to hold it oddly because it's sized for my hand very nicely. There were no 'small phone' habits for me to break. Other people? Smaller hands, smaller pockets, used to just keeping it in their pocket all the time and not taking it out when they sit down (and then trying to get it out of those smaller pockets while still seated when they need it instead of standing up to get it out), and that a lot of people simply don't treat the devices with much care fore even obvious stuff like
not throwing it onto a chair or bed or table. Yes it can bounce off and then hit the floor, and I have no idea why so many people don't consider that a possibility. 'I didn't know it could bounce.' There are people who can get away with not having a case because of some combination of luck, care, or lifestyle. I can't tell who exactly that is beforehand, in and in my experience most people can't tell if that's true of themselves beforehand either.
I do have a screen protector that I've changed once, and glad I did because it has saved me scratches.
/almost off topic rant.
