Impedance is one of a myriad of design trades. Like Bill I am not sure the majority of high-end speakers are lower than 8-ohm nominal but I have not looked. Some designs, electrostats, planer-dynamics and ribbons included, tend to exhibit lower impedance because they work under different principles and/or have less wire than a typical voice coil. Others choose to use less wire or put woofers (etc.) in parallel, and so forth. Also note dynamic impedance is rarely the same as the dc wire resistance. I am not aware of any strong arguments for or against lower-impedance speakers from a speaker design standpoint, but it is not my field.