i dont think retainers are really about spacing or smoothness..
they're about making the assembly process easier

no retainer = loose balls everywhere
BUT
loose bearings = more ballz to support the load, less wear on each ball
also
no retainer might mean a headset is less likely to get indexed from brinneling (the balls wear indentations in the headset cups from riding in a straight line all da time)
plus
removing the retainer and adding ballz from a worn headset with indexed steering can help the problem because the spacing is now switched up