For Mexico I do think it is worth it. It is a big country and you will spend days or weeks there. In Mexico technically it is compulsory to have liability insurance but many Mexicans do not have any insurance. Nobody asks if you have it- until you are involved in an accident.When you are in a collision wit a Mexican and the police are involved things will be a lot easier for you if you can show them your insurance policy and call the local rep for that company who has to come out and do his assesment. Without insurance you have a lot of explaining to do , the police will haul you to the station until things are settled ....could be hours could be days. No tourist wants to waste time and money doing that.
For the Central American countries BZ,NIC and CR demand and sell the insurance cover at the border. The other countries make no demands, , most locals seem uninsured, any problems are usually sorted out with the police and cash payouts by the "guilty" party.Having liability insurance would smoothe your way .
It is like buying a ticket in a lottery but with the objective of NOT having a "winning"