Diversity.  Each model brings a uniqueness in the way it is rigged and sculpted.  Some bend better, others have better symmetry, others still have better mesh and resolution.  Even though, each can equally be ethnically morphed into other nationalities, there are certain features that enhance or break the convincibility.  It comes down to what will get the job done better and how they perform in the final render.  

You will realize that many users actually render with multiple models just because they are different, not because they are better.  They each have their own quarks.  I prefer to work with V4, however I love Miki4 and I use Dawn occassionally.  But I also revisit some oldies from time to time because they are different.