I guess one of the real issues is that when you pick up the car it is absolutely impossible to spot all previous damages. Just last week I picked up a car in MUc and it was dark in the garage and the small car (Audi A4) was parked between two big SUVs. Hard enough to get in the car, impossible to do a thorough inspection of it like they do for the return. Even if you find some scratches, hard to get somebody to them sign off.
To avoid any issues I always opt for the 0,00€ excess extra insurance and that way I never have a problem but that raises the average price by a good amount of money which wouldn't be necessary if they would be fair.
Some smaller dents and scratches are normal wear of a car and should be covered by the normal rental price.