The general attitude by those who support the "No comment" response even when someone is totally innocent of any crime appears to be because of incrimination. The question is though, if a suspect is totally innocent, how can they possibly incriminate themself?
Kate McCann, by answering the questions truthfully, could have handed the police a motive on a plate. For example:
A: yes I did find Madeleine a handful at times. She could be difficult and demanding and very active, always jumping off the sofa.
A: yes, we sometimes gave her Calpol to help her settle at night, if she was under the weather
A: yes, I sometimes struggled coping with 3 children under 4.
etc, etc, etc
So, here we have a woman by her own admission found her child difficult, who struggled to cope with 3 young children (one of whom was a bit of a handful) and who gave them drugs to settle them. Say no more - there's your motive and means right there.