actually, i'd say insurance is the biggest selling point. if you fly at a chartered club without AMA, the chartered club could be held liable for damages as well as the landowner. that's why all chartered clubs require you to have AMA insurance. homeowner's policies don't generally pay out too much ($100,000 in the case discussed by Dave Brown). In a million dollar case, that would leave someone liable for the remaining $900,000.
my reasons are as folows: 99.9% because of insurance/clubs require it.
.1% magazine subscription.