I have towed for 19 years and, for me, regardless of state laws, an auxiliary brake system is mandatory. You will only need it once to justify the cost. I know of at least 3 times in all those years that I am convinced a collision would have resulted without aux brakes. None would have been my fault - there are crazy, inconsiderate, drivers out there that you sometimes cannot predict their actions no matter how alert you are.
But, who is at fault is not the issue. Avoiding an accident is.