I suppose, regardless of its value as evidence or not, a lie detector test can really help police exclude the people closest to a missing person quickly, from their investigations.
I don't know if I'm being simplistic, but I'd have thought commonsense would indicate that a person willing and eager to take a test, who encourages other relatives to take one too, would not raise suspicions in the way someone refusing would. Espeially if it's the parents of a child.
I'm sure I've seen several cases where people have been asked to take a test, agreed but it was never followed through. Police just seemed to be using it as a litmus test, to see how co-operative people were being. As they take a lot of time and money but can't be used as evidence, that seems a good strategy to me.