Perhaps the streamlining and symmetry of ship design somehow improves efficiency of the warp field or deflectors/shields surrounding the ship.
That's the usual excuse used in Trek -- that the shapes of starships are a function of their warp geometry. Although realistically, if there were some shapes that were better for warp geometry than others, then you'd expect most species' ships to converge around a common shape, the way different aquatic species like fish and cetaceans independently evolved into the same basic shape because it was the most hydrodynamic. Sure, there's room for some variation -- rays, squid, jellyfish -- but there's a degree of convergent evolution, form following function. The fact that most species' ship designs are wildly different from each other suggests that the main factor is simple aesthetics (which, of course, is true of their design in real life).