Well, I was thinkin bout this becuase of
a. It'll make the doors lighter
b. It'll be easier to mount an a-pillar bar without cutting into the plastic trim
c. it looks better imo
d. you can have the door open with your seatbelt on

just some things off the topof my head. Plus I just think door mounted seat belts look weird. Not to mention it it sturdier to have them on the car itself and not on the doors for safety purposes.