@BitFlipper: A protective cover isn't just for "protecting" your phone, it's also about personalizing the phone's appearence. (Looking at those Batman/Spiderman covers)
Remember when sales of Nokia 3210 goes hot 10+ years ago? The product isn't significantly different than other brands in terms of functionality. But being able to replace shell is a major selling point.
And the 30+ shops in a single shopping arcade selling different kinds of "protective covers" and/or replacement shells DO mean this is relevent marketing concern because people DO care how does their phone look like.