I think it really depends on how much time you want to dedicate to planning and booking your trip and how much control you feel you need to have.
You absolutely do not NEED a
Travel Agent. But there is a a lot to know about booking a WDW vacation which is why this board and so many others exist. Just choosing a resort can cause anxiety! Then there are the discounts. Do you have the time to check the boards every day, even several times a day so you know if a discount comes out for your trip? A good TA will keep on top of that and make changes to your reservations for you immediately if it is to your benefit. Some agents may offer to make Dining Reservations for you, and even help with a daily schedule of suggested things to do based on crowd levels and your needs.
But you lose all control over your reservation. If a discount comes out and you can't reach your TA, you can't make changes. If you decide you want to switch resorts or change your dates it must be done through the TA.
For many of us on the DIS the planning is almost more fun than the actual trip! (hey,

I said ALMOST!) People here can spend a year or more planning every little detail of their trip. Or can hop on occasionally to see what is going on as it gets closer to their trip so they have an idea of what to expect.
No matter which you choose just know that WDW isn't really as easy as your local amusement park. There are tons of things to see, do and experience and preplanning can really make a great vacation magical!