If you believe the OA serves no real purpose, I would say that either your lodge is not fulfilling its mission, or you aren't well informed about what the OA does in your council.
An active and thriving lodge provides a significant amount of service to the council, camps, and community, as well as financial support. At the National Order of the Arrow Conference last week, the new Chief Scout Executive addressed the attendees and threw around some statistics that indicated the OA provided over 1 million man-hours of service to councils and camps in 2011, and many more millions in direct f