We're talking about comedy v. preaching to the audience, not when to make a joke and when to make a serious point. They're completely different discussions.
And to the Stewart point, he was perfectly able to explain (using comedy) why he believed something without implying that you are lesser for disagreeing with it. Oliver seems to be able to do that at most 1/3 as much as Stewart. The remaining 2/3 is "not only am I right for believing this, and not only are you wrong and you should feel bad about yourself if you don't, but you're so wrong that I can't even comprehend how you could possibly disagree."
Not completely. If there was a subculture that he didn't get along with, he'd feel completely at home with completely steam rolling them. (I.e. Dungeons and Dragons) What I liked about Colbert is that he would try to jump into the culture and make absurd jokes with the community.
There's absolutely no reason someone has to adhere to some arbitrary ratio of comedy to seriousness (edit: or preachiness, etc).