but if the silly little pistons that only push in the middle of the pad are so bad, why would companies like AP and Alcon make them then?
Not questioning brake manufacturers product ranges, like everyone in manufacturing they make things that they can make money on. If that means using the same caliper castings with slightly different machining to fit different market niches then thats up to them.
But if you have small pistons pushing the middle of the pad, the pressure ditribution on the pad back face is uneven and poorly controlled. This can lead to many undesirable effects including:
Uneven pad heating, potentially leading localistised heating and warping of the disc
Uneven pad wear, shortening pad life
Pad edge flutter, noise generation mechanism, predominately higher frequencies
Pad spragging, leads to less efficent braking, poor pedal feel, pad sticking (tends to effect sliding fist more than opposed piston calipers)
Like I said it depends on pad size and piston size (also relative size in multi-piston calipers) and position.
If you want some more reading on the subject then try these:
Fieldhouse JD, Ashraf N “Observations of the disc/pad interface pressure distribution during variable braking conditions and its influence on brake noise”. XXVIIth μ Symposiumon 26 October 2007 in Bad Neuenahr, Germany
Published as a VDI-Fortschritt-Bericht (VDI-Report)
Fieldhouse JD, Ashraf N, Talbot CJ "Dynamic Centre of Pressure Measurements", 25th Annual Brake Colloquium & Engineering Display, SAE Ref 2007-01-3960 October 7-10, 2007, Orlando, FL, USA.
Also very interesting whilst on the subject of brake noise:
Fieldhouse JD “Noise Study Comparing Phenolic Pistons To Steel Pistons” 5th Sumitomo Bakelite Europe Friction Colloquium. Held every 2 years June 2007
My background is more in thermal judder and brake disc aerodynamics but John Fieldhouse was one of my PhD (The optimisation of Aerodynamic Properties of Ventilated Brake Discs) supervisor and is a bit of an expert in brake. He's done consultancy work with Alcon, Brembo, AP, Bentley Motors, BMW, Brabus, Nissan and Toyota to name a few and has published close to 100 papers on the subject
Sorry for the off topic.