I have towed 3 different Miatas (2000, 2010, 2012) over a total of 75,000 miles and have never used an Aux Brake system.
I have run emergency/panic stopping distance tests with and without the Miata and measure no difference in the stopping distance.
I have also helped more than 100 other Miata owners to set their cars up for towing and do not know more than a handful who do use an Aux Brake system.
Do what you are comfortable with.
Tim