Can some one tell my why BMW chose to use "single piston floating calipers" rather than 4 or 6 piston non-floating calipers on their E46 M3?

According to this web site:
http://www.hotrodheaven.com/tech/brakes/brakes1.htm
Non-floating piston are both lighter and more effective.

Has anyone changed their brake system to, say, a Brembo or an AP Racing, Non-floating 4 piston calipers? Can they describe the differences?