Heel/Toe is just another technique in the quiver of the drivers skills. When your coming in somewhere hard and fast and you wanna drop straight down a couple of cogs then you need to do it just to save the engine and give yourself beter control in braking. But if your in a long sweeper then heel and toeing might unsettle the car with torque pull or wheel locking/spinning. You'd make your own judgements when to use it, just as you would with left foot braking, handbraking, looking out the window, checking your hair in the mirror, giving that cute girl the eye! . . . . .
You know what I mean, it's down to the driver at the end of the day.