I disagree. Body language may, generally, be extremely informative and important. Let me give you an example:
Police officers are called to a house by a concerned neighbour. He suspects that the woman's abusive ex-husband is back, and asks the police to come by and check. The police knocks on the door, the woman opens. The officers ask: "Everything all right here, ma'am?" The woman replies: "Yes, of course". Based on the fear the officers can read from the woman's body language (posture, look in her eyes, tense lips, arms crossed stiffly, etc.), they realize that something is in fact very, very wrong. And they intervene.
Should e.g. police officers act solely upon information coming from people's mouth, they wouldn't be able to prevent much crime, I'll tell you.
I work as an interpreter, and for me the information gleaned from observing the client's body language is very important when I strive to convey the client's message as precisely as possible.
The fact that much body language (like for instance playing with one's hair) may be interpreted in a number of ways, does not make it useless. In combination with context and words uttered, a person's body language fills in the blanks and completes the picture. For the vast majority, body language is something which we rely upon even if we're not very aware of it.