And the fact remains you can proffer no good valid reason why alledged criminals would want an exhaustive review of all the evidence allegedly held against them. In the letter to David Cameron they state clearly their reasons for wanting such a review (to help find Madeleine and bring her back to her loving family, don’t give up on Madeleine) - now perhaps you can suggest a more malevolent reason. Bet you will struggle to or completely ignore this request.
According to Kate McCann, their investigation was hampered by a lack of information;
"The Portuguese authorities possess a great deal of material that was not included in the police file released into the public domain. The British police, too, hold information we do not have." [madeleine page 384]
They wanted access to that information, and seemed to think that a review would enable them to get it.
"Our own search, however, has significant limitations. Crucially, we do not have access to all of the information that has come in to the inquiry...The more data we can acquire, the more complete the picture will be and the stronger our chances of finding our daughter."
Why they thought a review would share information with them I don't know. Kate even suggested further action;
"If a review is declined, or indeed if no decision is ever made, we will be left with no alternative but to seek disclosure of all information possessed by the authorities relating to Madeleine’s disappearance."
I'm surprised that they would consider trying to get the authorities to disclose information. They had attempted that before with LP, and had failed. Neither the PJ, LP or the courts were going to ignore privacy and data protection laws just because the McCanns were the parents of Madeleine.
However, it seems that they wanted information to help with their own investigation.