We need to also remember, the saucer can be a life boat if the Star Drive must be jettisoned. And likewise, the Secondary hull serves as life boat if the saucer is uninhabitable. So there is another reason for having the airlocks there. And They're at the widest area of the secondary hull. But, I don't know if they were ever meant to be docked with anything but a service pod or shuttle. The gangway doors on the saucer, are much larger and I think allow entry to F and G decks. They're also rectangular so a shuttle or pod could not dock there.