There are many types of coverage offered on rental cars, none are really that cost effective from insuring yourself from claims, assuming you have personal coverage that transfers to rental cars.
Some coverage only covers the vehicle, others include a liability component if you are sued. None of the coverages will apply if you are found in violation of the contract (ie. unauthorized driver, drugs/alcohol, etc.). CDW coverage in particular covers the vehicle only, so if you are in a multi-vehicle accident, you're going to need to let your insurance company know anyways, in case you are sued.
Statistically speaking you are more likely to get into an accident within 5 miles of your home then you are driving across country or on vacation. You'd be better off ensuring you have sufficient and reasonable (not state minimum) insurance coverage on your own vehicle.
