Jin is no more subplot than any of the other main survivors.
So much of the early episodes featured prominent back story on him, and Sun, to the point that their story was just as important as Jack or Sawyer etc.
In fact, the simple fact that Sun was picked as one of the Oceanic six, cements that both she and Jin are critical to the story, just as we also have the whole is he or isnt he dead story, still hanging over us.
Jacob on the hand, is purely a plot device at this moment, not a central character.
I wouldnt call Charles Widmore a main character either, and he is just as much of an important character as Jacob at the moment.
