This case is so far from the norm that it ingenders its own conspiracies. Once the parents saw fit to employ private investigators contrary to Portuguese law and once their involvement with Metodo 3 and Correia was revealed the entire status of this case should have been reviewed.
If parents can add anything to a search and if they think they may be able to assist in a police investigation particularly in areas where intensive searches are required with limited manpower available I don't see a problem with that.
That is what Corrie McKeague's mother was able to do and if the searches she organised failed to find her son at least the ground covered by them was eliminated. Thus freeing up the police to carry out much needed official searches in probable sites volunteers have neither the access or the necessary skills to search.
Corrie's mother is a serving police officer with Police Scotland so knew what she was about.
The McCanns conducted a different type of search for Madeleine requiring different techniques and this involved private detectives.
I agree with you that Madeleine's case should have been reviewed, probably not for the same reason as you. Just as I think Corrie's case will probably be reviewed and lessons learnt.