most of the people who use the down elevator mix do so because it's a 'learned' habit from back in the days when channels were limited and the best way to get brakes was to put a trigger switch that was activated by the servo horn when down elevator was applied. these days you typically have more then enough channels to put the brakes on their own channel and use a switch or slider. some people still use the down elevator mix for taxi braking and have a switch or slider for parking brakes.