I wouldn't use a TA for Disney. We stay on DVC points, and I know more about booking what we like than most non-Disney TA's would. We got the V&A Chef's Table a few years ago because instead of leaving a message as asked, I called and called and called for 30 minutes. Then, when told I couldn't book yet because of our split stay, I asked them to please try anyway, and to their surprise, they were able to. I wouldn't expect a TA to be as pushy and assertive as I was there, but it worked for me.
We have a TA for cruises now, though. That's a different situation--it seems like my TA has much better deals, with onboard credit and a rebate after the cruise than I can get on my own.