I'm a TA, so I guess I'm biased!
Disney is a differant type of vacation than most travel products out there. If you want to use a TA for a Disney vacation, I always recomend a TA that specializes in Disney.
Dreams Unlimited, a sponser of these boards, are a great example. Because they book nothing but Disney, they are not only experts but have the time and resources to jump on codes and specials when they come out. A run-of-the-mill TA just doesn't have the time or resources to do this.
Like any other profession, there are good TA's and bad TA's. You have to shop around to find a good one. While it is possible to book your own vacations these days, many people don't have the time to spend hours online doing research and cost comparisons. So TA's still have a place in the travel industry.
I'm a corporate TA, and most of our clients barely have the time to call us to book an airilne ticket, let alone spend an hour online navigating airline sites and Expedia trying to do it themselves. And 9 time out of 10 we meet (and sometimes beat) the internet price anyway. So they save time and get the same low price. We even book Southwest tickets, and through their website, too!
So my advice? If you have the time and planing to you is fun, then by all means, go for booking it yourself! If you don't have much time or details drive you crazy, the go to DU or another Disney-only agency and let them do the work for you.