To my knowledge, the only way to get a WDW discount through AAA is to book a package.
As for the membership, I renew mine yearly. The auto benefits (free towing, discounts on service, etc.) are more than worth it. In addition, they provide discounts with lots of varying businesses (I got 30% off my eyeglasses at Lenscrafters for example) that more than make up for the cost.
You also get all sorts of free travel info (guide books, maps, TripTiks) & free travelers' checks.
I highly recommend it. Also, if you aren't currently a member, this year's Entertainment/Dine-a-Mate book has a coupon for new memberships.
If in doubt, stop into your local office & see what you get with your membership. I know that will convince you!