I think Carol Marcus was introduced because they need another woman who isn't a nurse or yeoman.
Plus she looks good in bra and knickers.
She did look good!
But FYI - they didn't use the nurse (who was actually a research biologist) or the yeoman (who has been used to good effect as a yeoman/security guard in the comics) or Pike's former first officer, or the leader of the Vulcan people.
She was brought in because of her relationship to Admiral Marcus but I'm glad we have another woman. Two is better than one.
I scratched my head at Spock's surprise at a new science officer. It's a research vessel with a crew of over 400 people and she is a lieutenant. They have more than one science officer of that rank on board (but apparently not more than two engineering officers).