Noise? What do you think???
Performance. BMW and most manufacturers spend a fair amount of money and time on exhaust systems. On the one hand, the EPA imposes strict regulations on OEM exhaust systems. On the other, manufacturers are engaged in a contest in the market place to produce the fastest most powerful machines in their class.
They have large, experienced engineering teams operating in expensive test facilities working on getting the most performance possible from their machines. I seriously doubt that someone working in his back yard going to an open system will positively affect performance without spending a lot of time tuning the engine to operate with it. More likely, he will do some expensive damage, probably in the form of burnt exhaust valves or holed pistons.