An FFDO is the last line of defense of the flight deck and while everyone hopes they'll never be tested, their presence places a significant obstacle in front of anyone trying to gain control. Removal of FFDOs means that if that door is breached, it's all over. $15 a flight seems a small price to pay for that bit of extra insurance.
There was one idiot FFDO who cranked one off into the side of the flight deck, but that was years ago. Most people have forgotten about him.