Negotiating your salary shows your employer that you are capable, confident and comfortable with what you have to offer. Those skills are the ones your employer will value when it comes to increasing your level of responsibility, trusting you with clients and promoting you.
Sandberg and Mayer are aberrations. This ginned up "fight" among feminists about those women is just that: ginned up by the news of two women who aren't like most of us. They are celebrity feminists, long after Gloria Steinem became one of the first, 40 years ago.