The reason so many mission statements are dumb is because the idea that every company needs one is dumb.
There are some organizations that need to decide who they are, why they are here, and what they do. Salt of the earth stuff. For them mission statements make sense.
Some companies are fueled by vision, and are attracted to vision statements. Specifically, what they aim to concretely manifest in the future.
Others are driven by their values, so doing their corporate values makes perfect sense.
Purely competitive firms can focus their day around their positioning statement.
Passionate firms can seek a statement that combines their passion with their customer's need. They are perfect candidates for a brand promise statement.
But does a company need all of the above to succeed?
I believe that marketers and branders must be pragmatic. Understand the culture, the need, and only then recommend the simplest thing that will work.