I agree with both sides, useful but not totally necessary.
I always think of a coworker friend several years ago, before these things were put in every car, had one in his honda van. he came in one day with a crunched rear.... he was watching the camera so intently that he missed a pole that was right there.
fast forward to my wife's newer model Buick, that beeps when you get close to something, then steady alarm tone when very close. It's better, but I still feel like it's possible to miss something. Still, I do think they are a good tool.
The one in my RV recently leaked. i never liked the thing because of low resolution and very poor night vision but it was still a little helpful sometimes, mainly for keeping an eye on the bikes on the rack back there....and also to see how close I am getting to that tree that i already knew was there form looking in the mirror....
Went to rvcams.com. they do have a great selection. I was hoping for something better than the OEM camera, but what they were going to sell me sounds like an exact replacement. So, I went on the hunt for something better. Struck out on anything better that would work with my existing wiring, but in the process found what looks to be the same thing on ebay for significantly less money than rvcams.com...so much so that while it kills me to not support local businesses, this thing shipped direct from China was so cheap that I'm not out enough to hurt even if it doesn't work. haven't had a chance to get it installed yet....