Power Window problems of the 2001 BMW 740

Driver's window regulator failed/broke with window in down position. This is the second time that particular regulator has failed. Last year, the right rear window regulator failed - I was able to raise that window manually. If the right rear window switch is activated, the motor will continue to run until the opposite direction is selected on the switch. Both failures were accompanied by a sound similar to breaking glass. I noted today when operating the left rear window (to minimize the buffeting at speed because of the left front window which is stuck half-way down), the same type of noise was evident when operated. I have purposely not operated the windows excessively because I have believed the window regulator to be a weak point on the seven series BMW's. A fellow CO-worker with the same year and model replaced both rear regulators on one visit to the dealership. I believe that this condition could potentially be serious and should be investigated, and not dismissed as routine wear & tear or maintenance.