Just wondering how you came up with that statement ?
Stones etc get flung up and hit the protector...they then get diverted higher up and usually hit windscreen or miss completetly.
Most insurance companies give you 1 free windscreen a year but wont give you a free bonnet respray.
I was referring to scratches specifically - not stone chips. That style of bonnet protector acts as a junk catcher - ie holds onto leaves, sticks, dirt and grime, which works like sand paper on your paint as it gets buffeted around. Even the mounting points will damage the paint. If the issue is stone chips, then a car bra (for long journeys) is
better or the adhesive protectors that are fixed directly to the paint, not allowing anything to be caught in between the guard and the paintwork. I've seen those old-fashioned plastic guards cause too much damage to make them any benefit at all.
I don't put anything on the front of my car. I look after the paint through regular and thorough detailing - I see stone chips as unfortunate and unavoidable. When they get to a point where I can't stand the sight of it, I'll respray (or sell the car).