I think it all boils down to how much time you have to put into it. You can book through Disney yourself, and you can watch for codes and get up early to call to see if they can be applied to your reservation. You can call and make your own dining reservations. If you spend much time here at the DIS, you have all the tools available to book the trip yourself.
However, sometimes you just don't have the time. I've got some family stuff going on right now that will be taking most of my spare time for the next few months. I wanted to go to Disney in January. I decided to try Dreams Unlimited this time, even though I've always booked my own trips directly through Disney. I don't have the time right now to watch for the different codes and get up early every day to call to see if something has come out. The best part is, since they get paid a commission by Disney, they don't cost ME anything extra. Even though it's a step out of my comfort zone, I'm so relieved to be leaving the "grunt work" to someone else for this trip. By the time it comes around in January, all I'll have to do is show up and relax.
So financially, there's not a lot of difference in booking yourself or booking through a Disney specialist. But as far as time and convenience, there's a big difference.