The thing with dual circuit brakes is it is "safety for the masses".
Touch wood, in 40 odd years of playing with cars, I've had numerous ones with single circuit brakes (I have 4 vehicles at the moment with that type of MC) and never had a brake failure, I have had one total brake failure with a dual circuit MC though.

However how many people do you know who never lift the bonnet on their cars, like ever?
Brakes will almost always give some indication something isn't right, whether the pedal starts to creep or the fluid level is dropping faster than one would anticipate, especially in an all drum set up.
Motorcycles, as far as I know, all still run single circuit master cylinders, as you have independent control of the front and rear brakes, if one end fails, the other will still work.
Dual circuit brakes in cars, are a safety system for people who mindlessly make their way through life giving zero f#cks about how a car works, as long as it starts, it's "all good".
