You probably have looked at it details already, but some "free" rental coverages have exclusion of certain types of vehicle. I have read some exclude more expensive cars (not even talking about "exotic" types). Plus, the max coverage is often less than Amex. Obviously, it's no surprise that paid coverage is better than free coverage, but there may be times it's worthwhile to pay for that added coverage.
LAX
The 2 situations where it might make sense to take the paid coverage is if it is a type of car that is not covered by the free rental coverage OR if you don't want to have to deal with your own insurance company and have your rates raised since the free insurance is only secondary. However, if you don't have collision insurance on your own personal insurance then you don't have to worry about the free insurance being secondary as they won't be going to your personal insurance anyway.