The only real additional benefit you get would be if you booked through AAA. They have some extras included, such as special fireworks viewing areas in each of the parks, and the AAA diamond parking pass, which gets you a spot closer to the entrance, if there are any left. I believe they also offer some discounts, but if you have a Disney Visa, you get them anyway.
I've used them a few times and it priced out exactly the same. I just told the agent exactly what I wanted and she booked it that way.
You don't have to buy your airfare from them if you prefer to buy your own.