Apple clearly choosed for a very appealing design rather than building a solid phone but I don't see how that can be called a design fault. Any buyer looking at an iPhone 4 in a shop has no difficulty to figure out that it is more prone to damage than a thick and ugly water and dust proof phone.
I don't think that my HTC desire would come unharmed from a fall on a concrete stairway but I wouldn't blame HTC for that. And, yes, some cases are worse than no case at all, but that's the same for many phone models. I'll stick with my silicone case around my HTC desire. Not the best look, but it absorbs the shocks way better than hard cases and will not scratch the phone (unless some sand finds his way between the case and the phone, so, clean the inside regularly !!!).