It's not an issue of whether the TA can get better deals, it's more an issue of knowledge and timing, imho. This is the first trip that I'm using a ta and I love it. I get to do the fun things in the planning and decision making process while she makes the ressies, changes and updates when there's a special. Plus those tas seem to know when the discounts come out a bit before we do.
Even if you use a ta, you should still do research but you have someone that will help you out with the decisions and make all those phone calls for you.
If you use a ta, make sure you use a disney ta. I like Your Magical Journey, others on the dis board will recommend other tas like
Dreams Unlimited. I'm sure there are good and bad experiences with different companies and I'm sure others will chime in.
Whatever you do, you'll have a great time at wdw.