I disagree. My M3 is the same power, whether or not the 'Sport' button is on or off. Pressing it just makes the throttle more responsive.
That's like saying why have a traction control button. It's personal preference as to whether you have it on or off.
Pretty sure the newer ones, and the m5, have a button for more power rather then just throttle response?
TCS and a button to give you a wallop more BHP are a bit different, but I kind of see your point. I just can't see why get a car with x bhp then drive it with less.