Consistancy is very important. Think for example, of the US navy... everything pretty much looks the same. Big ships, boxy, grey, same white lettering in front.
Think of it this way, everything is consistant, because the same basic problems need to be mastered. All ships need a hull, powerplants, a method of navigation, weaponry. Due to the ocean and materials at hand, all ships have a basic prinicple behind them, and hardly varies from ship to ship.
Look, for instance at nuclear subs versus conventional subs. Why the difference in design? Different theory and principles, mashed together with better technology. Why should this be any different in space? Why would there be a long flat fighter next to a short stubby one? Why would one be almost completely vertical as compared to a horizontally orientated fighter? There is no orientation in space except when given a reference point.
Speaking of reference points, wouldn't it make sense if a navy had a particular method of docking, so that all fighters did so in the same manner? That way all docks would be universal. This would filter into the general design of the ship.
Space will have its own priorites, just like air and water do. For this reason all jets have wings, all helicopters rotors. These are like boomers compared to hunter-killer subs, different missions to accomplish. Same with the destroyer versus the aircraft carrier. Vastly different purposes, different designs, many recognizable features shared between them, the same basic principle.
It will be true for space when we finally get there.
I like what Kalfireth said about evolution. That is the difference between designs in the same class of ships.