My list of good reasons to join:
1) The knowledge on this board is easily worth the cost. Find the solution to your problems and don't waste money on ineffective solutions. The problem you are having is not new and someone has fixed it before. Find out how they did it and save time and money.
2) Guys like knucklehead_61 who open up their shops and help to wrench on anyone's rig. When I joined a few years ago, I asked for help and had a senior member agree to do free welding on my rig and told me the best parts to upgrade next. Even though I was not a member, his experience saved me money and got me on the right track.
(Thanks Letterman!)
3) Discounts at many great vendors will save you more money than the annual membership cost.
4) Special runs for red-letter names; last years Rubicon run at Sierra Fest is a good example.
5) You get a cool

sticker for your rig which gives you instant street and trail cred.
6)

donates significant money to groups that protect the lands we wheel on and keeps them open. Last year

and the Sierra Chapter promised over $2k to groups like Friends-of-Fordyce, CORVA, and others that keep local trails open.
7) Even the runs for anyone are a good reason to sign up - last weekend we took out a group of 12 guys (most of whom were newbies) and got everyone home safe. There was some carnage (broken axle shaft, bad alternator due to mud, overheated CPS) but everyone had fun, got off the trail, and home. There is a good atmosphere in this club and everyone looks out for each other. There is always someone to go wheeling with.
8) Red letter names get access to more sections of the website.
9) You can poke jokes at guys like JoseJalapeno and srmitchell if you are a member.
10) Last but not least - I hear that we are getting JoseJalapeno bobble-head dolls for the red-letter names.
(Disclaimer: this isn't true yet, but let's keep our hopes up. Send PMs to VetsXJ to make this happen.)
OK, so the last one is made up but I really wanted to get to 10.