As far as the trip itself, unless the TA can get some sort of discount or incentives, I don't see that there should be any reason to use a TA. The process took about 45 minutes on the phone, however. I guess you could avoid that with an agent
Well, of course I'm a little biased being a TA, but the advantages would be:
1) Many of us have experienced AbDs already (booking my third!) and have had feedback from many of our clients, so can offer helpful suggestions.
2) If the agent is a Disney specialist, he or she will often know of any discounts or promotions that can be applied to your trip.
3) We're available pretty much 24/7 for questions and concerns, and you'd have an email address to contact us when things pop into your mind at 3am.

4) I give really good gifts (as do many other agencies!) - lol!
5) We need the money more than Disney does. ;-)
I have a friend/client who's very anal about booking her own trips, and does three or four AbD reservations each year. She normally will call AbD for the initial booking and then tell them to have me handle the details from then on in. It's saves her all the time-consuming follow-up work (airline details, missing birthdates, passport numbers, etc.) and we get to talk more than we would otherwise -- a bonus for me.
As with everything, it's a personal choice. I never used a TA until I became one.
Dotti