I always book it myself on the phone. It is easy and I know I'm in control of my reservation and if I need to, I pick up the phone and call Disney myself. If a discount comes up, I call Disney and know I've done it. I love researching all of my ADR's etc., and then I do them online. And now there are other online things going on, I want control. I change ADR's around, so I just do it.
Some people claim their reason for using AAA agent is because of some kind of diamond parking and maybe some other perk, which none of that means anything to me, so if that is important to you, it could be a reason to go to them. But if you're just seeking a AAA discount, you can also call yourself and see if the AAA discount is available, just like at any hotel.
Some people have wonderful travel agents who are probably very knowledgeable and quick and etc., but others have not been as fortunate.
I know I have the best interest of anyone for my vacation and so I prefer and enjoy handling it. If you are not that type of person, you may prefer to let someone handle it. Just be sure to make sure you keep up with them to get what you want. But I figure if I would have to call them, why not just call myself? I don't think you are going to get any more discounts on prices with them than you are going to get by booking yourself. Sometimes they throw in extra perks like a gift basket at you resort or like I said, diamond parking.
In regards to calling or online, I always call for the initial reservation and any impotant changes like discount promotions or rate or room changes, but then I do the other stuff like ADR's and if you do online check in or now I guess this magic band stuff (which I haven't done yet).