What you say is certainly true when he is interviewing entertainers, but I do think the parent is correct: if he's got a serious guest on and they are discussing an issue Stewart cares about, he really lets them use the time effectively. He mostly stays out of the way and just helps them stay on topic.
Promoting his agenda through guests is his reward, I suppose, for stumbling through so many interviews with vapid celebrities.
Yeah, the less antagonistic serious interviews go okay. That's why I was so specific about guests that he can't find respect for.
I also don't think he is really stumbling through the celebrity interviews, he's playing himself as above them. When his (lack of) relationship with the guest makes it so that won't work, the interview goes along just fine.
Promoting his agenda through guests is his reward, I suppose, for stumbling through so many interviews with vapid celebrities.