I think 7 pieces is the optimal size for a class set with a 6pc bonus. It gives more choice (for Natalya for instance, one build might get the 6 pc bonus by using her weapon and dropping the ring, while another might keep both weapon and ring but drop the pants in favor of Hexing.)
More importantly however, 7pcs makes it easier to hit that viable 6/4 set combo. Examples include "Firerasha", "Delrasha" (pre-nerf), and Zunitooth. We could get so many more combinations like that if all sets had a 7th piece. Vyrbirds, Innwuko, Railiana, Rolakkhan... the sky's the limit.
If you all could combine any 6pc bonus with any 4pc bonus in a class what would you come up with?