Asking a question of a speaker is not rocket science, wouldn't you agree? Good questions give speakers an opportunity to deepen your understanding of their area of expertise. Ask for additional information on the subject or about something related to the topic that was not covered during the presentation. Ask for expansion on a key point or clarification of a different point or even repetition of another point. Most speakers are happy to do so, especially if a key point from the presentation is part of the question - shows the person asking was really listening.
But how many times have you known immediately when an audience member opening their mouth has no intention of actually asking a question of the speaker? When I hear sentences starting with "Don't you think...?" or "Are you trying to tell us...?" or "Isn't it a fact...?", I shut down instantly. Here we go - another audience expert about to show the rest of us how smart he or she is. Q&A? Nope, because the "A" goes missing, replaced by showboating, hot air & tangents. Do these dunces know how obvious they are?
In my experience, this kind of nonsense is so prevalent I'm not unhappy when Q&As are dispensed with. Give the expert the extra air time. And save this crab some aggravation.