I'm fairly sure it's because of the planned future possibilities of ships carrying other ships; or even having an in-system specialized ship parked somewhere. FSD class only affects jump range, so being able to downsize to a 2D can save a decent bit of tonnage on a ship like an Asp or Cobra, improving combat performance by that much more.i wonder why we are even allowed to fit smaller fsds. It's a little odd since even one size down means you can't leave the system anymore. it seems like an oversight considering we can't fit anything else undersized except shields, and even then only 2 or 3 sizes smaller.