Sometimes I sit in a coffee shop and very often see people just using their side mirrors (no rear cam, not looking back) ... scary.
I say this as someone who learned to drive and drove for a long time with no cameras. People saying this are just usually just used to mirrors and never bothered to take the time to learn to switch to the cameras for backwards maneuvering, or their cars just are not equiped with the latest tech.
Using the rear-view cam in this context is definitely a lot safer (esp for kids!), though you really should also look around as well, the side mirrors are more for driving, not parking-lot type situations, and have a really limited field of view.
What I meant was: people backing up just using their side/rear mirrors are scary. They don't have good spatial awareness of what's around them. A rear-view cam for them adds a whole lot of safety though.