It's about dollars and egos
It's about $$$$$$! Quit pretending like it isn't. BMW is saving money. Do you get 90% of the physics with fewer pistons? Yes. Are they just as objectively effective in most circumstances? Yes. Does Porsche put big reds on their turbos primarily because they recognize ego plays a huge roll in buying a porsche? absolutely (how many porsches are driven on the track?)
So what do you get from more pistons/stiffer calipers? You gain the ability to modulate the brakes more precisely at the edge of lockup (or the edge of abs). Does this matter with street tires on the street with ABS? Not really.
Does it matter with R-comps that generate significantly higher friction rates and skid transitions than most ABS systems are designed to handle? absolutely. You don't want to be into the ABS with slicks as the car will hop around like a teenager on ecstasy at an oakenfold concert. Not a particularly enjoyable experience on the front straight at sebring at 140mph...
More pistons = more control, but we are talking about the edge of the envelope where far less than 1% of these cars ever get.
Porsche saves money too, look at that IDIOTIC (when it comes to the track) wet/dry sump japanese boxster engine they've been putting in the 911 for 10 years.
The equation e=mv^2 has NOTHING to do with proper brake modulation at the edge.
Again, we are talking about very subtle subjective differences in feel and the ability to control the car more precisely at the extreme. Hardly worth the cost for the 99% of porsches and BMWs that never get tracked, but it's a great marketing tool for the balding stockbroker nonetheless.
You are both right, it just depends on which point you are more focused (the ego issue or the brake feel issue).