When I rent a car in the US, they rarely ever try to sell me the redundant coverages already covered by your own insurance and credit card. Perhaps they are reading my scowling face or have a rental profile on file;)
If my wife is at the counter, they always automatically add it, and simply bury it when they ask her to initial 10 items.
Redundant coverage is the easiest money they make, especially at the cut rate outlets.
It something you need in Jamaica, Ireland, and a few other countries, and as Bart says, it is easier if there's an incident, but paying
40-100% of the rental cost for a week's insurance is absurd.
If there is an incident however, they will charge you first, then your credit card will pay. I've had it happen (a truck's spare tire fell off in front of me and I ran over it doing about $2500 damage) process took about 20 minutes via phone and fax and Amex reimbursed me in about 2 weeks.