He says this because a decent single pivot brake with good pads is typically more than enough out back. I have 2 road bikes with single pivot rear brakes. Oe is a vintage monoplanar and the other is a modern record caliper paired with a dual pivot on the front. I use the same pads in each and the bikes share wheelsets. Both bikes have enough braking power to lock up the rear wheel without requiring a lot of hand strength. A more powerful brake would just be added weight.