well laughter is important and conversations with some laughs in between are quite lovely. what i am talking about is laughter while speaking and it's different from being delighted while you speak about something. this applies specially when the audience is diverse, with different mother tongues and different languages.
mixing laughter with speech makes what's being said harder to understand, not just that, putting your thoughts into words is a serious affair, and laughter dilutes that. sure you can have phases of laughter and serious talk one after the other but never together.