I have no doubt that mission statements are real at some companies, and that they influence results in a positive way.

After making my comment, I made a short search, and posted this: Sex, Lies and Mission Statements. http://news.ycombinator.com/item?id=5352298

It's from 1997. Things may have improved, I don't follow HR research. But it's at least a description of how things can go wrong, and probably still do.

Summary: Most managers report that their company's mission statement is ineffective, and many report that it's counter productive. Employee involvement in creating the mission statement is correlated with positive influence on behavior, yet that involvement is rare. Free coffee cups for everyone!