Good question. LED lights largely resolve the issue of adding extra load to signal circuits. You still have the issue of how the bike’s wiring will be affected if a problem develops in the trailer’s wiring. By wiring direct, you’re adding a lot of extra “exposure”.

If the bike is relatively simple with respect to electronic controls and sensors, the result of a short in the trailer’s wiring or a problem at the plug will probably just pop a fuse in the affected circuit(s). However, more touring bikes are moving to monitored circuit architectures (CAN-BUS) which are much more sensitive to unexpected variations in bike circuits. If the trailer (or more likely, the plug) develops an intermittent problem that isn’t enough to cause an all-out short, it can still be something that an active circuit monitoring system would pick up and flag as a “fault.” That might mean something as harmless as an unexplained trouble light on the console, or if the control module thought it was serious enough, it might disable the bike to avoid further damage. A relay system would electrically isolate the trailer so even if load isn’t an issue, a potential problem in the trailer would not affect the bike.

Again, older model bikes, not so sensitive to this. Bikes coming on line now and in the future, far more sensitive to issues like this.