I've used Stephanie at Small World travel to book both packages and room-only. I have also booked by myself, but it is nice to be able to fire off an email and come home to "Your reservation number is #xyz" when I don't have the time to call for myself. I also know that she will watch for any codes or discounts that come out later, so I don't have to. If I want to make changes, I would have to go through her, though, not Disney. (I'm pretty sure....)
It's not necessary and I doubt they can get rooms when you can't but using a TA has some things going for it. All depends on what you want.