Bike Anti-Porn - When Things Go Wrong

And what looks like a dinner plate sized rotor on the rear of that tandem in the background.

Back brakes are actually useful on tandems, and it's handy to have one with a high capacity to dissipate energy when going down hill. The reduced lever force implied by a big rotor also reduces hand fatigue on long descents. It might be unusual to see a huge disc rotor on a tandem, but it has sound physics behind it.