I installed the sensors myself a couple of weeks ago. Installation of the wiring harness is a real pain, so I understand the dealer cost to install. I did it at the same time as I installed a trailer hitch to be used for a bike rack. I did not install trailer wiring since I don't plan on pulling anything. I have mixed feelings on the usefulness of the backup sensors. If the Clarity had a camera that could sense distance (Phase shift pixels or similar technology), and had some decent image processing software, the backup sensors would be redundant. It doesn't. Where I work, the parking garage has the walls painted white, and you can't tell distance in the camera until the car shakes when it hits the wall. This morning, as I pulled into the garage, the driver of car in front of me did not have valid ID to open the gate. I had to back up to let him out of the garage. While I was doing this, a pedestrian ran about a foot behind my car. I was concentrating on the car in front of me, the concrete walls to each side and heard the sensors beep loudly. Did the backup sensors help? I don't know.
I do think the answer to "Why do you need backup sensors when you have a camera?" is the same answer to "Why do you need Automated Emergency Braking?" Drivers get distracted.