Currently, the only blocks with Edge counterparts offer little functionality beyond health, mass and volume, this means that designs with large edge or corner blocks suffer from significant "deadweight" blocks.
Normally this is countered by removing the large edge pieces for smaller ones with internal blocks inside, such as thrusters to move the additional mass, however this leads to high block counts and lower performance. Edge blocks of things such as Crew Quarters, Cargo Bays, Assemblys etc, would allow for a high-performance block count design that retains functionality in these blocks.
Secondly, Edge blocks of additional block types will make outer appearances better, expanding build possibilities.
As this image of one of my ships shows, There is an abrupt halt in the "windowed effect" on the side of the vessels bow, thanks to the lack of a crew quarters edge, this is also true on other parts of the vessel with corners present. The above issue also results in that part of the vessel consisting of over 25% the total blockcount, while less then 20% of the mass.
I don't personally believe that aesthetic design choices like this would result in an imbalance, especially compared to the "block stacked cube" build, and would allow for better creativity through better enabling non-cuboid shaped designs.