Cherry does not have to put on the pretense of being a Soc when she is with Ponyboy. Just as Pony is not fully embedded in his status as a Greaser, and can easily walk between the two worlds through his manner and intelligence, Cherry is also one who represents the moderate Soc; not fully engulfed in the lifestyle and social expectations of her group. The two are, in effect, 'outsiders' themselves--meaning that they are on the fringe of their own social groups as a result of their values and personalities. They are kindred spirits, but are likely doomed to walk their teen days separated by respective class boundaries.
From a literary standpoint, Cherry's presence also serves as the keyhole through which we see the humanity and problems of the antagonists--the Socs. Because we need to see the author's theme that 'things are tough all over,' the author likely needed a character that gives us a glimpse into the Soc world and was herself sympathetic. She was flawed, as evidenced by her statement that she might not greet Pony if she were to see him in school, but we see she was struggling with essentially the same issues as Pony. Instead of a jarring change of point of view in the novel, the author was able to give us information from the other perspective by making Cherry accessible to us. It was necessary for her to be able to go between both camps.