I have been backing up trailers the old way for 45 years and as a rule don't use backup assist. However, for people that aren't set in their ways it can be useful. And I suspect that if I were to tow smaller trailers I would find it much more useful. Big trailers that are easy to see and respond slowly are easy to back up. Small utility and Wave Runner trailers can be very challenging to see and they jackknife in a heartbeat. The backup assist can see the trailer all of the time, respond very quickly for that type of trailer and let the driver focus on where they want the rig to go and not running over anything else in the process. I see the value more for those trailers.
I cringe every time I see the commercial with somebody backing up a big toy hauler in a gas station. I have had too many people jump into the blind spot behind the trailer in those situations for me to even consider backing up more than a few feet.