You pinpoint the whole reason that it
should be seniority. If people sign up just to get tickets, screw them. If you give the same benefit equally to both old and new members, you end up with brokers & scalpers and people like the dude a few posts up who bought six memberships just to get six sets of presale codes. That's ridiculous.
Sure, it's not as fair to people who only recently became rabid fans. I've been a fan for a long time, but have only been a member since 2005, and I know there are thousands who've been there longer than me, so I'm nowhere near the top of the list. Sure, there aren't a ton of great benefits to being a fan club member. But if you're signing up for a one-time thing simply to get a presale code, IMO it's perfectly justified that you're on the bottom of the pile.