Actually it is VERY worth it. You'll find that most societies require paid memberships, how else are they going to be able to afford advertising?
The IBC is a society of betta enthusiasts and breeders. They develop the standards for each fish as well as promote ETHICAL breeding practices. They also have a species maintenance program designed to keep the 70 odd Betta species alive and thriving.
When you join the IBC (which BTW is only $20 a year.. I spend more on frozen fish food) you gain access to lots of information as well as direct access to experienced betta keepers. People who know how to breed and treat diseases and can tell you why your betta exhibits a certain behavior. You can learn the history of the species and how to preserve and better it at the same time. If your a breeder you can learn the standards and begin showing your fish and know what to look for in a GOOD betta and how to propagate good fish in successive generations.