First, I did join the AMA so that I wouldn't be criticised for criticising the organization.
The insurance is, as Air5 said, secondary to your homeowner's insurance, but it's worth having if someone requires you to provide proof of insurance to operate from their property. The insurance is a standard aircraft liability policy with all AMA members as named insured. There is no section in the policy where I can find any discrimination of model aircraft type.
Historically, the AMA is intolerant of new technology, openly hostile to personal drones and absolutely horrified at FPV. Autonomous flight is not in their vocabulary. Many AMA fields still won't permit drones or FPV flight. If you fly a balsa wood model at the end of a pair of 50 ft fishing lines at an AMA field, you are their kind of member. Otherwise their "rules" are virtually impossible to comply with completely. The AMA comment to the Part 107 NPRM was horribly mismanaged and may have done more harm to the hobby than good, but that's another topic.
In my opinion, yes, you should join the AMA. even if you only fly your Phantom from your back yard. The insurance has untested benefit (and you are unlikely to ever need it), but the membership card add some level, though microscopic, of proof to the local gendarme that you are a responsible flyer.