Simple. The McCanns' greatest hope is that pressure is brought to bear on the Portuguese courts to declare there isn't enough evidence to try them. But in order to go back to having a normal life afterwards, the public has to believe they had nothing to do with Madeleine's disappearance in the first place.
I think the PR team for the McCanns is working on two objectives at once. The first one is to negate the impact of the forensic evidence- downplay the importance of the DNA (soiled diapers and sandal sweat), criticize the cadaver and blood dogs (chicken carcasses and smelly trash) and come up with "logical" reasons (death smell on Kate's clothes and Cuddle Cat) for some of the findings.
The second is to try the case in the media. By having family and friends speak up about the first objective as well as leak new witness statements, they create favorable opinions about the McCanns' guilt, or rather, lack of it. You'll notice there's very little actual fence-sitting; people either believe they had something to do with harming Madeleine or they didn't. I also saw something written on a blog about how the McCanns are pushing polar opposites. Stranger abduction vs. premeditated murder, with no gray in between so that public opinion has to settle on abduction. I mean, what kind of loving parent purposely murders their darling 3-year-old child and then chillingly goes out to eat as if nothing happened!
Throw in a few Maddie sightings to create reasonable doubt, and you have the masses screaming that the McCanns are being smeared and arrested for something they didn't do. Count on high-powered people they know to get the charges dropped and they can reclaim their middle-class lifestyle and pretend it was all a bad dream, even if they are one child short. That's why the parents care.