Yes, of course, obey the law.
MOST states do NOT have any law requiring brakes on a towed motor vehicle.
MOST states DO, however, have laws requiring brakes on a TRAILER. Many states have a legal definition of a TRAILER that does NOT match a towed motor vehicle.
Many states (and the Federal Motor Vehicle Safety Standards) (FMVSS) have a braking performance law that states a combination of vehicles must be able to stop within a certain distance (usually 40 or 45 feet), from a certain speed (usually 20 MPH) on a level, dry, clean, hard surface.
The laws requiring a breakaway system also apply to trailers, not towed motor vehicles in many states.
Yes, common sense tells me that a braking system on a towed vehicle is a good idea.
Common sense also tells me that if such a braking system is an absolute necessity, in the Nanny state of today, there WOULD be a law requiring such a system, and there would be standards that would have to be met by the manufacturers, and the systems would be thoroughly tested and approved by an independent testing agency. The Insurance Institute for Highway Safety would be all over the issue like stink on a cattle feedlot!
All that having been said, I towed vehicles on dollys, and Jeeps four down, for many years (including many trips over the Continental Divide) without a braking system of any kind. In all those years, I never had to make an "emergency stop" when towing.
I have done considerable research into the laws of my state, and the only law I have to worry about is the Braking Performance Law (MCA 61-9-312).
Other states, of course, may be different, and Reciprocity Agreements do not apply AFAIK.