HI Hakn,
Happy new year to you as well.
My belief is that high eff. speakers :a) allow the use of low powered SET amps, and b) produce significanlty lesser distortion at normal listening volumes. Other than that, I don't see any purpose. It is also true that it is much harder to get a flat frequency response from high eff speakers, i think becuase of the limitations of materials that can be used to make high eff. speakers.
Just my beliefs,as of today (they may change asi learn more) mostly empirical.
-akhilesh