Door stuck closed are also a safety hazard as they can hinder evacuation in case of fire or other emergency and make it more difficult for rescue teams to enter the train. Imagine a packed railroad car full of smoke, very bad visibility and panicking people inside. A stuck door can make the difference between life and death.
I have ridden BART most workdays for the past 7 years. I have encountered trains with stuck doors a several times, and on none of those occasions was the train put out of service. In fact, they even have pre-printed stickers that say something to the effect of "This door is not working; please use other door." I have seen these perhaps half a dozen times since I began riding BART regularly.
Note that these doors were stuck _closed_. It would be an obvious safety hazard if the door was stuck open - in that case I can well imagine that the operator would not move the train from the station.