Blender Buying - Square vs. Round Container
I can think of two reasons to prefer square containers :


They store in less space for the same volume, when you're dealing with dozens of them (as the smoothie shops likely are)
The key to a good blending is that the inside is not round -- you need baffles and such to keep the blender from forming a single, smooth vortex.  The square containers would help to make baffles more effective by providing larger areas with slower moving liquid away from the center of the vortex.