As with everything else, there are some who are fortunate to avoid any cracks, scratches or dings during their ownership.
However, after dropping my Blackberry 8310 down two flights of stairs, I have always had a rugged case on my smartphones for the last 15 years.
The Otterbox Defender Pro while quite bulky in its earliest iterations has managed to slim down drastically.
It is important to make a distinction between the Defender Pro and Defender Pro XT which are both slimmer that the regular Defender case with the latter being slimmer but harder to install than the Defender Pro.
It's all a matter of personal preference and comfort with the risk of damaging your phone especially with repair costs even with AppleCare +TL.
Finally, these phones are glass and metal and cost alot. Even with AppleCare + TL, it isn't prudent IMHO to not protect such an investment. In addition, my smartphones have always been in pristine condition at trade-in time fetching a higher value.