I am in favour of products that do have a provable positive benefit on towing safety, and trailer stability systems like ATC and similar products are well documented as working, Notwithstanding their obvious benefits, what they are actually doing is correcting a situation that should never occur if the driver is doing his job properly.
We have seen a few reports from some contributors where, they have stated their outfit was difficult to control but having fitted one of these systems they find that can tow quicker! This is definitely not what these systems are designed to do, They are not there to correct avoidable problems like mismatched or poorly loaded outfits, or to tow beyond the speed limits set for such outfits.
It has already been stated that they should be considered to be an additional safety device like air bags and seat belts to catch the very odd occasion where things catch a driver out.
The idea has been raised of why aren't ATC's a standard EU requirement, The problem is the present EU regulations do not recognise towing of caravans as any different to other trailers, and that includes commercial units. ATC is manufacture specific solution, and could not be fitted all trailers. However that does not mean in the future (inside or outside of the EU) such devises may not become mandatory, as the evidence base grows about their effectiveness.
I know its not been mentioned so far in this thread, there has been no proven case for the fitting of tyre retaining bands to wheels. The biggest bit of hypocrisy is when some one makes big claims for the benefits of tyre bands on a trailer but they do not have them fitted to their tow vehicle! In my opinion a tyre pressure monitoring system is far more valuable and informative.