I bolded the question.
The harm would be that if he was awake, he had information about the case. If he was asleep, then there is no information. The harm is that when interviewing a child, the child might inadvertently let something slip like JonBenet walking up the stairs that night on her own. The harm is that something he said might contradict what the parents said. When you have to stick to a story, a story that is questioned because the daughter has something like...oh, I don't know...pineapple in her stomach and all you claim is that she was carried to bed from the car, then you don't want your son changing the story. All of a sudden, there are questions that your story can't answer so you have to cover. The best thing to do is to stick with the original story and deny, deny, deny.
I understand why John was angry when he discovered that Burke was interviewed at the Whites. Now he has to discover what Burke said. Had Burke not had secrets, this may have been a very different case.
God, I'm starting to sound cynical. I've come not to believe what the Ramsey's said about what happened that night and the next morning. I don't need to bicker about the discrepancies in their testimony because I believe they're being deceptive. One of my favorite quotes from Judge Judy, "If it doesn't make sense, it isn't true." That's not always true, but it's a good rule. Jonbenet had pineapple in her duodenum. There was pineapple in the fridge on the first floor and on the kitchen table. There are specific timelines for digestion that some people here have tried to discredit. Let's not get into an argument here because I'm convinced that the pineapple absolutely puts the Ramseys in a lie. The only thing I question now is what that lie means. I don't have an answer for that. I only have theories and not one of them, not one, gives me any satisfaction.
The 'harm' is that Burke could have inadvertently exposed a lie. From there, John and Patsy would have to come-up with another story and this house of cards could have come-a-tumblin-down.
All in my own opinion.