All phones are designed to be used with a case, but the iPhone more so than others. As a side note, I just ordered an Otterbox Defender for mine, it is semi-hideous, but they offer the best protection. It's literally irreplaceable, so I don't want it getting screwed up. Not saying everyone should get an Otterbox, but some form of case that has some type of impact energy management in it is necessary for the iPhone.
It's not an issue of not being able to take care of your phone, it's the fact that phones have a habit of sliding off of and out of things, and hitting the concrete. Mine has hit tile and other hard surfaces several times, and thanks to it's dual-layer Incipio case, it's fine. If it didn't have the case, it would have spider-web cracking on the corners. It wasn't an issue in the flip phone days, mine hit the ground or went down stairs a few times, but now it is an issue, since these phones can't handle that on their own. There's a reason that virtually all of the early OG iPhones had spider-web cracking on their screens- there really were no cases as first, and they would hit the ground from time to time.
The iPad also needs a screen protector and a back case. Incipio makes a nice one that allows the use of the smart cover on the front with the case on the back.
I've seen way too many cracked iPhones, and smartphones in general, to be so stupid as to not put a case on my iPhone.