Short of one of the big, bulky and awkward "ruggedized" smart phones, there is not a single smart phone on the market that I would not put in a case. Anything slim/thin and covered in glass (even the toughest glass) with expensive electronics in it is inherently somewhat easy to break. Thin, slim, light weight and feature packed means compromises in robustness.
But I hardly consider them useless just because I feel I need a case on mine. It is, in the end, nothing more than a telephone with some additional communication and computation features. I only own one at all because I need a phone (my iPhone being my only phone), not to make some kind of fashion or artistic statement. Function trumps form 100% of the time when it comes to any tool, and as nice as I think my iPhone is, it is ultimately just a communications tool.
But, compared to handsets of the 1990's, things have already come such a huge way, its anybody's guess what some engineer or company will come up with next year, or sometime later?