Before I was a GM I was a Supporter for a long while, almost since I discovered how much I liked the game. A huge part of both why I really liked the game and why I became a Supporter was with the realization that being a Supporter gives virtually no advantage whatsoever (other than the lessons learned from playing meaningless PL games). There are no stats, skill or cash bonuses, hidden skills, cash payouts, attendance bonuses, etc. that Supporters get. In other words, there is no way to buy your way to an advantage here like in so many other games. You just pay if like the game and want to enjoy it more with fancy logos, uniform numbers, etc. I would not stick around in a game for long if membership or having to pay for anything made some teams better than others, those aren't so much fun. I'd rather support a game that's fun.
That's my totally unsolicited two cents on why I became a Supporter.
Don't ask what sort of Chunks they are, you probably don't want to know. Blowing Chunks since Season 4!