You're correct - my dad has supplied coke plants before. Boke bottles start out thick and test-tube shaped, with the threads on top and a stright tubular body. They are blown up into final proportions later.
The shape might not be a huge problem, but it might not be trivial either. To blow the preform into the bottle's final shape, it is reheated, and then air is forced into the preform, causing it to expand to fill the mold containing it. The issue you would see with a bottle like this is that when the preform is reheated, the plastic in the wall tends to flow depending on the final shape.
When you have sharp corners, like those displayed, it is difficult to maintain an even wall thickness on the final bottle. So you either have to use more plastic than you want to ensure a minimum wall thickness is kept, or risk having thin spots, which hurts the structural integrity of the bottle.
There is a reason that you don't see many square bottles. The ones you do see, like Fiji, are really thick to account for this. You can get away with thick bottles when you are charging $4 for a bottle of water. Less so when charging a quarter that price for soda.
the problems are as swombat and Zev pointed out
1) missing cokes iconic shape
2) uncomfortable to hold square bottles.
So why not give the bottle some of that iconic shape in the middle? Go from square to a round with a wave through it?
I'm sure that geometrically this isn't a huge problem with modern manufacturing techniques.
I'm unsure how difficult it would be to get a shape like that to crush easily, but maybe it's possible.
I suspect the offset of the cap may cause problems with packability, as it seems the bottles need to be stacked in a proper order.