Think about it. Advertising, or the 'art of persuasion' is all about telling lies, or at least 'overcoming people's resistance'.
If the plain brown caremelised liquid known as 'coca-cola' was sold in a plain tin, there is no way it would cost >£1 a tin, nor would Coca-Cola have a larger profit that the GDP of the entire state of Mexico.
Extrapolate this into criminal law. Imagine there has been a crime committed. You are interviewed by the police.
If you are innocent, you just tell the truth. If guilty, you have to come up with an 'innocent' script. This involves acting. You have to imagine how an innocent person would act in your position, and what they might say.
Thus we saw a situation in the Kercher murder of all of the other room mates and witnesses immediately seeking an attorney. Knox and Sollecito imagined that an innocent person wouldn't do this, so casually pretended they didn't need one and declined to appoint one.
Now, because a guilty person aiming to evade justice has to dream up a pack of lies, in effect, of course statement analysis is useful, as we can pick out contradictions, changes in stories (Raff changed his five times), changes in reported emotions. One minute Knox claims she was frantic with worry about Mez, banging on her door and shouting her name, in her email to the world, yet all witnesses at the scene when the police arrived report she was entirely laid back about the locked door, even going so far as to telling Battistelli the door was often locked.
The problem with lying is that people forget their lies, so when we examine Knox and Sollecito's babblings, and compare their book, tv and film narratives with what they told the police and the courts, we see a fascinating phenomenon of one lie after another and a fake alibi.
Crini explained in his submissions that a fake alibi was a piece of evidence in itself.
Thus, statement analysis is invaluable to courts and this is exactly what barristers are getting at when they cross-examine. They are aiming to highlight contradictions, fabrications and anomalies, which will go into their closing submissions.