Without a wheelie bar, if the RC driver gives the RC too much throttle the sudden acceleration and torque could cause the front of the RC to raise up too far, too fast and make it flip upside down.
With the front wheels off the ground you can't steer and without a wheelie bar to control the angle of the wheelie or prevent a wheelie some RCs may tend to lift off the ground too far or at the wrong time so that the driver can't turn the vehicle and loses control.
