The audience listens, presumably learns, and eventually gets to ask questions. Sometimes panels are more spirited than others. Some moderators are better than others at giving all the panelists a chance to talk. But no moderator can stop someone who is bound and determined to be a jerk from getting his way. Some panelists have gotten bitter at only publishing in small presses or in small magazines and want to rail against the state of publishing; some have gotten so successful, they now think they're God's gift to the genre; some have no real knowledge of the topic, but don't let that stop them from laying down the law to the rest of the panel; some are just plain hung over. At bigger conventions like Worldcons, I have been sadly disappointed in learning the ugly truth about writers I had admired for years. I have walked out of panels saying to myself, "Well, that's one writer whose books I'll never buy again."
I can only conclude these folks either never had the ability to look at themselves from other people's perspectives, or if they did, they've lost it. We all have egos, but most of us have some perspective, too.
Sometimes a little distance is a good thing.