For the FBA, you do not have to be a member to compete, but you have to be a member to be in the Triple Crown Contests and also to have your points count toward Team of the Year consideration.
I prefer to cook in sanctioned contests where you have certified judges and not just the average guy off the street determining who wins. Too much time, effort, and money committed to have it just a crapshoot with no standards in place. Having a board that oversees all of this stuff takes money, of course.
Also, with the FBA you get a years subscription to the National Barbecue News when you join.
Last but not least, I try to do what I can to encourage the growth of this "sport" and I feel that supporting the organization helps in that effort.