I have used a WDW specialized TA sometimes, and sometimes I book myself.
Ears To You has been great. Using a TA isn't cheaper, they can get you the same rates as you would get booking directly.
Then benefits as I see it are that:
*They can do the Disney Math, when WDW is putting out multiple discounts, IE: free dining or room discount, or the 4/3 promo, they will number crunch and figure out the best deal for you.
*They know sometimes a tad ahead, when new discounts are coming out, and will automatically be on the phone with WDW, getting you the discount if it is applicable to your vacation. So they keep an eye on discounts, doing the leg work for you.
yes, they can save you $, thru their Knowledge. They can also suggest touring plans, and restaurants you may enjoy, and if you want, make your ADRs for you.
Another thing i like is that if you want to make a change, or add something on, it's just an e mail away, you don't have to phone WDW and stay on hold waiting. Just fire off an E mail to TA. Works for making payments too. I'll just e mail TA and ask her to make a payment for me.