I think it's all relative to your specific situation. Personally, I don't use one for Disney. However, I do see the value of using a Travel Agent for their knowledge base and convenience. If you're comfortable enough to navigate around the "world" with the planning and booking aspect of it (and know to continue to check for rate drops and promos after the time of initial booking) and feel that you understand the "in's and out's" - then I guess it comes down to cost.
If you are getting a lower rate through a TA, then use the TA. If the rate is lower through Disney, then book direct.
There are also a few intangibles, based again on your specifics. It's akin to asking whether you should buy "Travel Insurance". A lot of people don't. They figure "I'm not cancelling" - but what happens if they get a flat tire on the way to the airport and miss their flight. Trip Interruption comes in pretty handy in that case. I had a friend who refused to buy TI even for international travel. I asked him one day: "You know if you get sick, break your leg, or have any other type of medical mishap - your insurance coverage generally doesn't cover you if you're out of the country - TI does". He's been buying TI ever since. Those are the type of things where a knowledgeable agent comes in handy.
