But didn't I just make a case (heh) that it's not only about drop fragility?

It's a cheap sacrificial shell that adds utility. My examples were non-slip and grime from a public table. Maybe add scratches and impact damage in there too because every material is susceptible to that, and avoiding them helps me resell it.

There's no way to build that into a phone. I don't want rubber non-slip bumpers on my phone or a built-in clawed up plastic shell when I can have them in a swappable shell.