Adm. Marcus postponed the attack on the Enterprise when he saw Carol on board, but went on with his orders once she was removed. She really played no pertinent role on the Vengeance, and ended up being beamed right back. I felt it would have been a more powerful scene -- and given more meaning to Adm Marcus' convictions -- if he had chosen to sacrifice Carol in lieu of his desire for total war. It would have also provided an antithesis to the theme of "family first." Scotty nullified the attack by disabling the Vengeance anyway, so it's not like she would have been killed. Plus, it would have really revealed how much of a bastard he was.