I remember when I was in OHIO for the parkour Horizons/Generations workshop.  a young boy there asked a question to Stephane Vigroux "have you ver had to use your Parkour skills to help some one in danger?"  Stephane simply said "no" (keep in mind he does keep his answers short because A: English is a second language to him, and B; that night he was tired.  Quickly after his statement I believe Kaz Uma took the mike and said "When we would train with David or when the Yamakasi would train, we would often create scenerios in our heads to help us overcome our fear of a dangerous physical challenge that we may have been about to do."  "After a certain amount of time that method does not work any more and you have to do task for the sake of doing it for you, because you choose to do it"   in different arts their is always the thing that you practice, soem of those thinsg help you create schemas to aide in other areas of life.  From Aikido I orginally learned how to role away and turn energy into a circle, from break dance I learned how to move in almost all dynamic situations on flat ground, and in parkour i have learned that I need to be full of life and a love for moveing and training hard every day all the time.  If those things can help me save someone when they can't save themselves then so be it.  I have rarely seen a martial artist have to fight outside the Dojo, I have rarely seen a dancer move in a dynamic manner other than for the sake of dancing, But in parkour I have seen it more done mainly because you use it where you practice it "outside and on urban structures"   I think it's just the playing field in which we are comfortable and so we use it most often in that situation because are most often in that situation. 

MURG