First, I want to say I largely agree with you.
Second, I want to say that the 10% of a company's values/missions that aren't tautological marketing catch phrases are actually -incredibly- important, and can directly contribute to a company's success. The fact 90% deviate is more to do with companies not realizing how important it is to not just be marketing fluff, than any intrinsic value or lack thereof of having such a statement.
Fundamentally, a company has many decisions where they have to choose A or B, not both. Oftentimes these decisions are made at a level the CEO or similar has no insight into. To be able to ensure people make the right decisions and are all pulling in the same direction, a culture has to be adopted. Mission statements, value statements, etc, are the only tool the CEO really has to shape that culture, beyond simply hiring into the roles directly underneath him (which is hard to do without insight into what is going on beneath -them-). The Culture Code has a great segment in it about how Johnson & Johnson's credo (specifically, "We believe our first responsibility is to the doctors, nurses and patients, to mothers and fathers and all others who use our products and services") led to its handling of the cyanide in the tylenol incident, which saved the product line (when every analyst was like "that's done, no one will trust Tylenol again"), as well as introduced the now ubiqitous tamper proof packaging.