I think I am quite handy at parking, I learn't to drive aged 17 in an old D reg transit and could stick that away no problem
What I think parking cameras are useful for are for objects that are simply obscured from view by mirrors and conventional head turning. I have nudged a plastic post in a car park in the zed reversing because it was painted dark grey (with a yellow cap but really may as well have been invisible for what use it was) and the post was in the centre of the back of the parking spot and about two foot high so unless you clocked it in the dark as you passed the spot you got no chance, unless you got a reversing camera/sensor. I got out and checked the car (no damage or marks ) but looked at all these posts in the car park which were covered in various paint colour marks where everyone had done the same thing - it wasnt a car park of useless drivers it was the fact that the situation dictated that a camera would have stopped the impact as it would have had more opportunity to see it. Even Dan with his god given parking talent may have come close to hitting one me thinks
So, yes, most people can park to a good degree, some not at all, some better than others - are parking sensors useful? I think so, on occasion. Would I pay a load extra for them, probably not. If two identical cars were up for sale for the same amount of money and one had them and another car didnt then I might be swayed by the one that had them, surely