Almost everyone I've ever seen has a case on their iPhone which makes it bulkier so I don't think the bulkiness argument holds any water. Apparently many users must actually want the phone to be bulkier. It's easier to hold securely, makes it possible to hold it against your shoulder (almost anyway). This race to the bottom as far as thinness goes is such a strange fad. My phone is about 1cm thick and that's as thin as I want it to be. Sure super thin looks sleek but that's as far as it goes.
This new case design looks really sweet I think. Makes the phone look just about right. Maybe they should offer a version without the headphone adapter for those that don't care about that and just want extra battery life and a little thickness.