I can't find the link, but know this was discussed on these boards in the past couple of years. As I recall, there were concerns expressed about not only the low fuel, but also supplies including food, water and oxygen being in short supply with an added passenger. I think there was also mention of how the added weight of an additional person might throw off the flight path of the shuttle, so it might miss its target or need a course correction using additional fuel. And there was concern that Lois's seat wasn't designed to be used during lift-off, so she might not have survived.
