Ever wonder how many times you step on the brake pedal in a year?
I disconnect mine when not in use. This was suggested by Jordan when I visited his facilities. It was expressed more with the idea of longivity of the controller rather than any thought to there being a premature failure.
I don't think it's likely to fail during the warranty period or any time soon with normal driving. However, I suspect that if you have high mileage but don't tow but a small portion of the miles, it would be a good idea to disconnect it.
One thing that disconnecting does do for you is it prevents the display from activating when you step on the brake when you aren't towing. Some have found it annoying and installed a power switch.
I guess it also depends on how often you tow (inconvenience) and how long you might want to keep the controller.