When you see kids jumping off buildings, there is a definite potential (certainty?) for long term and possible short term damage. Parkour is really more about going over small walls, railings, sets of up to 6-8 stairs, obstacles in the 5 to 6 foot range can be taken fairly quickly, obstacles up to 8 or even 10 should always be done from a "hanging drop" and not just jumping off.

When we teach our boot camp, the first 6 hour sessions are dedicated just to landing and rolling, I'd suggest at least 3X that in practice before even jumping off stuff at your own head height, and no matter how much practice, there is still always potential for injury. The idea isn't so much that you can never get hurt, but that by training and conditioning diligently you are much better prepared to safely and quickly overcome obstacles, however it doesn't mean that you can or should just start to take on obstacles of any size, past a certain point there is almost no positive benefit and very little practical application.