Two things mainly: 1) The narrower base make it less stable when setting down somewhere. 2) It's wider than it needs to be. For any given volume, if you take a cylinder and make the base narrower, the upper portion has to be made wider. 3) (I lied) There's no need for it other than for aesthetics. They don't even look that comfortable to hold in your hand, especially with some insulated gloves on.

Plus I have no idea how well those odd shapes work in most bottle carriers. They might be ok, but I'm skeptical.