She already was one of the Big Three in ST09. McCoy wasn't nearly as important a character to ST09 as Uhura was.
needs to go back and read Tara's amazing breakdown of the structure of the movie and how McCoy and Uhura are MIRRORS of each other in terms of character and function in the "47 Klingon Ships and Uhura" thread. . . http://www.trekbbs.com/showthread.ph...01#post3668901
She proves that neither McCoy nor Uhura are more important than the other, and they probably even have the same number of scenes and lines if you want to be anal about it. . . It is a brilliant piece of analysis. . . print it out, memorize it, tape it to your computer and when another Uhura vs. the "Big Three" topic comes up, read it again. . . seriously. . .
Hopefully the writers will continue making ALL of the characters integral to the plot. . .