When they deny people boarding when a flight is closed, they will get an ear full of people that have watched in disgust of U/J getting on the plane. These people will be under the impression that they deserve the same courtesy. I could see people yelling, sceaming, kicking at the agents. "The plane is taxing but it hasn't made it to the runway yet. Pull it back now and get us on it."

There is a reason for rules, because if you do it for one person, everyone else would want the same treatment. It doesn't matter what the circumstances are.