BMX bikes were originally designed for racing on short, technical dirt circuit inspired by motocross racing, hence the MX in BMX. The racing is dwonright explosive, and the only time the racers sit on their saddles is at the end of the race. The racing is a lot like an out of saddle sprint through out the whole race.
I wasn't sure about BMX on a paved course, but the racing at the Olympics, which in 2012 was the first time for BMX, sold me on it. It was exciting to watch.
Freestyle BMX developed when guys like Bob Haro took their BMX bikes and rode them at skateparks. Freestyle is pretty diverse and includes the trick riding shown in the vids that Ragman posted.
Kids don't ride BMX bikes for their appropriatness for commuting, they ride them because, as bomerang said, they're the rage.
The pressure on the knees is ridiculous
after a short while riding.
Haro is a venerable brand.