It's been used as part of the "proof" that Knox's statements to the police were entirely her own idea and she was under absolutely no pressure from the police.
My theory is that it was all part of the plan. The investigators had already decided that Knox and Sollecito were guilty. That is how they seem do business in Perugia. They decide who is guilty and then go find the evidence to convict instead of the other way around.
They worked on Sollecito, falsely claiming the had irrefutable video evidence that Knox was at the cottage on the night of the murder. Sollecito knew she was with him, but threatened with 30 years in jail as an accomplice to murder, he changed his story. He wasn't willing to spend decades in jail for a girl he had known less than two weeks. Too bad he didn't know it was a classic police trick. The police got him to change his story, and therefore he is a proven liar.
And you know in your heart that he is a murderer. Because nobody but a murder ever changes their story under stress, or has inconsistencies. The rest of us have perfect memory, perfect recall and perfect consistency.
Amanda was told the same lie about evidence putting her at the scene. She was also told that her boyfriend had said she left. (He may not have even said this yet.) They hammered her about the text message to Lumumba. The meaning is clear as day, but Mignini sees conspiracies everywhere he looks. After hours of stress and lack of sleep, she cracks up sometime around 5 AM. Falling for another old police trick, "can you imagine what it would have been like if you had been there".
When Lumumba is arrested, he is worked over the same way. But he is older and a lot tougher than these two college students. He knows how the game is played. Patrick is also lucky he was at his bar that night. If he had been home with only his wife to vouch for him, everyone would assume she was just lying to protect her husband.
Lumumba had another problem. Do you remember the cell phone evidence placing his in the vicinity of the cottage? We know it was a false indication, but if he was the suspect it would have been presented as proof. It would have played well with the jury.