My own opinion might be a little biased, but I def think so. No matter who you are or what you do with your phone, you're bound to drop it at some point, with it sliding across the tiled floor or pavement, without a case that going to be dinged and scratched for as long as you use it. At least with a case on, its some what protected. I'm not saying everyone should go get an otterbox, but a case does help.
And sometimes its just a tee shirt you know, everyone's using the same black or white iPhone, they all look the same, don't you want something that looks more "you"?