Some cranky personal opinion follows; not speaking as a moderator.
It was actually the original intention for a skilled pilot to be able to operate the vehicle more efficiently, as simulated by an increase in ISP. It was dropped because the adherents of strict realism went absolutely bonkers on the forum, insisting that the operator of a vehicle could not alter the fuel efficiency of its engines. They would not accept that the ISP change would only be a simplified representation of better steering and operation. In fact, they made such a fuss that the concept was dropped, so that was the end of that idea.