It's not just that. The questions from the audience are often questions I didn't even think to ask but are very enlightening. The questions taken in the hallway

It's not just that. The questions from the audience are often questions I didn't even think to ask but are very enlightening. The questions taken in the hallway won't necessarily be heard/overheard nor will the answers like they would when the presenter is in a space designed to make sure everyone hears. When a speaker takes questions from the audience, they are putting themselves out there. If you're selling something, sure stay on message and don't take questions. However, if you're teaching, you should take questions where everyone can hear the questions and answers so your "students" learn what you came to communicate.