There is a couple of cons to it, which have to be monitored.
Increased heat, the motor is always doing something, so if you are pushing temps already, running drag brakes will make you run hotter.
Battery life, again, the motor is always doing something, so while sitting on the start grid your car has the brakes applied, make sure you have enough battery to spare the added needs.
Tire wear, very slight, but there is increased tire wear due to constant loads.